Why is my fileprovider location in the files app grey (inactive)

I have a working fileprovider extension but if I go to the photo album, for example, and share a photo and choose 'Save to Files' my files provider location appears in the list but is grey and inactive so it cannot be chosen.

What do I need to do so that it is enabled?

As mentioned in this thread The typeIdentifier for the root container must be set but it must also be writable or the files app will not be able to copy files into your container even if sub folders are writable.

Why is my fileprovider location in the files app grey (inactive)
 
 
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