Hello Viktor, this error means that the appropriate module for creep analysis was not loaded. This is typically done by adding the following keyword /M CCStructuresCreep when starting ATENA for instance by AtenaConsole. Normally this is done automatically if you start ATENA directly from GiD. The question is how are you executing ATENA? Try to add this command when starting ATENA. It should look something like this:
%AtenaStudio64% /M CCStructuresCreep /execute /catch_fp_instructs /o /inp "file_name.inp"