It is currently not possible to make statements with properties of data type Lexeme or Form. However this did work earlier today. There are existing statements (for example: https://www.wikidata.org/wiki/Special:WhatLinksHere/Property:P5191). I can reproduce it on Wikidata and test.
Steps to reproduce:
- go to Lexeme
- start adding a new statement with a property of data type Lexeme (e.g. "derived from")
- select the property by mouse or keyboard
- see value input field not appear and instead see message show up as shown in screenshot