Diagnosis
You are editing a step loop and have chosen Select loop end.

System Response
The system uses the current cursor position to define the size of theloop block as a rectangular screen section and immediately tries tocreate a minimal step loop of two repetitions.
Since there is not enough space below the loop block for anyrepetition, the system rejects the loop block in this size.

Procedure
Place the cursor so that the system can create at least two looprepetitions and repeat the function.
If there is not enough space for a repetition below the loop block, youmust leave step loop processing with F3=Back and create the requiredspace on the fullscreen first, e.g. by marking and moving.