Check out this page to see if any common issues can be resolved. With numbers for easy referencing!

1. Tags + Space Hierarchy

A lot of issues are caused by Scenes or tags being used incorrectly or not at all. Mona needs the following applied exactly like this in order to function correctly, so make sure to keep the following in mind.
Space Summary
The next image is a summary of the object types and the applicable states of each. Some of these are compulsory as well, so make sure these are set accordingly.
Other considerations to make in your hierarchy

2. 'More than one root object' error!

This means that there is probably an object outside of the Space Scene/object hierarchy. If you reduce all your folders to the top gameobject layer you should get something like this :
Reduced hierarchy should look like this
If there are any objects under the Space/Portals/Artifacts scene, then make sure to add them under the Space/Portals/Artifacts gameobject.

3. The camera is stuck/doesn't move in Mona!

Make sure that your space doesn't have a camera in the scene. Either delete or disable the camera before building your Space to Playground Files.

4. 'There is a problem with the Collider on one of your Canvases'

Canvases MUST have a mesh collider on the Unique Name Gameobject that has the Canvas tag (Not the CanvasPrefab parent object). If it has any other collider type, or no collider at all, it will throw an error on uploading to the playground files on your wallet.

5. My space doesn't update when I build to Playground!

Make sure to delete your Playground Files after testing. Sometimes it will not overwrite the files when you build them.

6. 'You're missing a Space/Portals/Artifacts Layer' error!

The first object under each scene is the 'layer' in which all your objects need to be under. The Tags (in green) should be as follows :
Object tag in hierarchy
Make sure that there is only one of each green tag in the space. Any duplicates will throw an error. Other tags (Portals/Artifacts/Canvases) use different tags.

7. My avatar can't 'see' my Portal/Artifact/Canvas!

You may have deleted the Collider on the gameobject under the PortalPrefab that has the Unique Name and Portal/Artifact/Canvas tag attached (as per the documentation) .
Make sure there is a collider for Mona to see that it is a Portal/Artifact/Canvas.

8. 'Duplicate Portal/Artifact/Canvas' Error in my build!

This means that that two or more UniqueName gameobjects (with their respective tag) under different Portal/Artifact/Canvas Prefabs has the same name. Use the QA button to give you hints to which ones inside Unity.

9. There is a very specific pink material on some/all of my objects!

This is a shader issue usually related to render pipelines. You may be using a shader on your material that the inbuilt render pipeline (URP and HDRP are not compatible with Mona) is not compatible with.
If you are porting your space from either URP or HDRP, played with these settings by mistake, or added an incompatible shader type from the asset store then you will need to reset the material or render pipeline to the Inbuilt Pipeline.

10. My artifacts / canvases / dynamic objects are really dark!

This could be that your light probes do not have enough light information in them to make them the right brightness level.

11. My Avatar falls through the ground!

You will need to add colliders to all objects you want the avatar to walk on, or not walk through. Box Colliders are most performant, but if you need a mesh collider make sure to optimise the mesh and use that instead of the base mesh.

12. Playground Files won't upload (with no error)!

First thing is to check the web console for errors : - Edge > Settings in top right / More Tools / Developer Tools - Chrome > Settings in top right / More Tools / Developer Tools
If an error is 'Nullreference : Object Reference not set to an instance of an object' then there may be a broken connection between objects.
A common issue is that Artifacts, Portals or Canvases (the Unique name object with the respective tag) does not have a collider and it needs it.
It could be by deleting prefabs without unpacking/deleting the references in the scene, renaming or importing a object that is different in some way to what Unity is expecting, references to objects in Unity Animation, or a reactor reference perhaps.
Basically, something in your scene is looking for something else in you scene, and not finding it.

13. dataOffset !=file.GetPosition() Error in Unity!

This happens sometimes when your hard drive that unity uses for temp files is full. Check your main drive and make sure there is enough space.

14. My mesh collider mesh looks different from what I exported!

Make sure to close your mesh before export. Mesh colliders should be closed objects (in that there are no 'holes' in the object). Otherwise there may be differences in your mesh in your 3D asset creation tool and what imports into unity.
Also make sure to apply all transforms before exporting.

15. My space works in playground but has trouble uploading to the Wallet!

It could be that your internet connection times out before it can upload, resulting in the following error. Make sure that you have a fast enough internet connection to upload your space without issue. Sometimes this may just be a temporary issue, and may work in an hour or two.
Example issue

16. I have a 'Looks like the "Space' file is missing' error when I upload my Playable files!

This error will happen when you have moved or renamed your space scene in the project file outside of the original location, breaking a lot of the internal references that Mona is looking for.
Error in question
This can be fixed by copying the Space.unity.meta file from an alternative build (found in the Space location) and copying it to the same location once you have sent the Space file to it's original location.

17. I can't seem to sign into or install Unity Hub!

Occasionally, the Unity Hub can be a little problematic for some uses. One thread that may help is the following, as it has links to earlier version of the Unity Hub.