It appears from casual use that labels must have a minimum of 3 and maximum of 49 characters.
Although use of the word between can sometimes have exclusive boundaries, the more common case in forms like this is inclusive: i.e., both 2-character and 50-character labels should be accepted; or the error text should read
Must be between 3 and 49 characters.
This is a problem in many places, not just here. At some point a sweep should be done to make this consistent across the suite.