Hello Sasa,
It could be possible that the CHECK DATA is already running
Check the currently active tasks in the database:
From DB50 - Go to -> Kernel Threads -> Task Manager
See if the Task type ChkData is already running.
From DBMCLI -> Connect to database as control user
run "show active"
If a ChkData task is already active on the database, you need to wait until it is finished to start another one.
Regards,Yashwanth