Hi, I had the same error and I just figured out what was the problem.
I am currently configuring my first ever server and using unraid for the first time.
My setup is an old server rack mount server with 2 1tb samsung SSDs.
So i configured my array with 1 parity disk and 1 data disk.
I noticed that in the main tab my data disk had this message under the size column: UNMOUNTABLE: UNSUPPORTED PARTITION LAYOUT.
I formated the disk with unraid (under Array Operation) and it solved my issue.
I was then able to create a share normally!
Hope this helps.