Brief description of the problem
You cant set any compression argument to the “extra mount options” field.
Since you can set the “compression” field to no, it should be allowed to set compression to the “extra” field.
OR
Add ZSTD to the compression options 
Web-UI screenshot
Error Traceback provided on the Web-UI
Traceback (most recent call last):
File “/opt/rockstor/eggs/gunicorn-19.7.1-py2.7.egg/gunicorn/workers/sync.py”, line 68, in run_for_one
self.accept(listener)
File “/opt/rockstor/eggs/gunicorn-19.7.1-py2.7.egg/gunicorn/workers/sync.py”, line 27, in accept
client, addr = listener.accept()
File “/usr/lib64/python2.7/socket.py”, line 202, in accept
sock, addr = self._sock.accept()
error: [Errno 11] Resource temporarily unavailable