To reproduce from scratch in 2019.3.5f1:
Add a 3D object to the scene in Hierarchy.
Add a custom tag and assign it to the object.
Drag the object to the Assets folder to create a prefab.
Delete the object from the Hierarchy.
Edit the list of tags by either editing a different object or clicking Edit Menu -> Project Settings... -> Tags and Layers.
Delete the tag from the list of tags (can't delete tag if there is still an object in the hierarchy using that tag).
Select the prefab and this will throw the error.

The issue is that if you edit the prefab, the tag appears to be valid because it shows as being the selected tag. If you then check your Tags and Layers it appears there too but I think that under the hood it hasn't actually been added to the list.

To fix:
Select the model that is causing the issue and change the tag to something else. The original tag will still be in the list so you can choose the original tag again and everything works again with no errors.

Cheers!

This is happening to me - 2019.3.10f1