You should be able to complete that without a problem using a Single-Line Text property as long as you haven’t used your available unique values, which you indicated you have not.
Does changing the name/internal value of the property allow you to save this with that rule or are you unable to create ANY unique values?
So, gave it a different label and was able to save. Wanted Client ID and was able to save it as Client Unique ID. Then went back and edited Client Unique ID to Client ID and it saved. So not sure what is up but, net/net I got where I need to be. Thank you!