The GEWTT1 is part of the GE wind turbine model and the error happens if the model in DLL format is not loaded before the dynamic run starts.
User defined models are usually provided in *.obj or *.lib binary format that needs to be compiled using a fortran compiler compatible with the PSSe version, using a compile process recommended by PTI, to create a DLL file. This final DLL file is loaded in PSSe before performing a dynamic run to include such user defined model into the run. Some user defined models are provided in DLL format, bypassing the compiling process. More related information can be found at posting "Create User dll and access it in PSS/E"
Use "psspy.addmodellibrary" to load the apropriate DLL file to the dynamic run.