If further assistance is required, please post to the NetBox mailing list.
But on my home test instance, I get the same error as you:
<class 'AttributeError'>
'str' object has no attribute 'available_columns'
Python version: 3.8.10
NetBox version: 3.0.0
Also, it seems the stylesheet isn't applied. That's something that has been observed by someone else independently:
https://github.com/netbox-community/netbox/issues/7082#issuecomment-909066758
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