Primavera P6 Import Event UCVFT-1399-D

Good Day,

I have Primavera P6 Professional Release 18.8.21 (Build#: 37039). I imported a MS Project Schedule into a new P6 Project. I filled in necessary activity codes and exported the project to an .XER. I then opened the current Overall Project Schedule being maintained and attempted to add the converted MS Project segment by importing the XER. The process added the WBS elements and came to a halt, displaying the Event above before the activities could be imported. I absolutely have to incorporate these 150 activities into the OPS for maintenance. Any guidance available to