Create a new character style does not work (plus stays grey)

Hello,

I am just doing my first steps using Adobe XD CC Version 54.

In my document I created several kinds of text and turned them into a new character style.

Now I cannot create a new character style anymore because the "+" stays grey. It does not matter if I mark the text or choose the text frame.

In a new document I can create one single character style but after that the + stays grey again.

Could you kindly give me some hint?
Thank you very much and kind regards