define new brush to use it as watermark - error code "no pixel selected"

Good morning

after a few hours I need your help:

I want to defne a brush as watermark - as described in some videos.

I create new image - define pixel - enter Text with text editor  - and go to "bearbeiten" - "define brush" - and then I always get the error code: " Brush cannot be defined as no pixels were selected"

No matter what I do - it just does not work - I tried " select all" , cut and copy, reduce my work to one Ebene, nothing helps.

It did work once - but I was not able to reproduce it.

Who can help me??

Thank you!

eva