It’s not forbidden, it’s not supported 
So, you need to do a research, how to allow the NFS client to make a support of flock.
By the way, I tried hashstore with a CIFS mount in storj-up and it’s working so far. Of course, not so much data there, it’s not prod, but I was unable to got this issue so far.
For SMB I was forced to use nobrl option to make SQLite happy, this seems also allowed me to avoid the flock issue.
However, I saw a PR, that should solve this issue for NFS too: https://review.dev.storj.io/c/storj/storj/+/16046