Hello Team,
It has definitely something to do with your List/library.
Can you create a blank new list or library and check if you are able to reproduce the error over there.
The least privilege required for Request sign off is Contribute, so technically it should work but can you also assign someone a Full Control permission and check once.
I just checked this in my environment and it is working just fine, so something to do with your environment in particular.
These are troubleshooting steps to find the root cause of the issue. Also please let check if you are using Government GCC cloud, I can see that in some forums it has been mentioned that GCC clouds do not support Request sign off yet.
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