Just installed the Trial. When generating a report this error comes up: Something went wrong: Cannot call method "getRange" of null

This school year I've been using the Google Classroom mark book. For my class I have four categories, each one of them counting as 25% of the final grade. Each assignment in a given category is weighted according to the number of points. The Google Classroom mark book correctly calculates the final grade. I am trialing the GradeBook for Google Sheets, but after importing the same classroom into the gradebook, and setting up the categories, total points etc. the final grade is way lower than it should be. Why?

For three of the four classes that I have, there is a vast discrepancy between the Final Grade that appears in GradeBook for Google Sheets and the Overall Grade in Google Classroom. For the fourth class, the grade is close but not exact. Please advise soonest.

I've already noticed that there is a big gap between the grades on Google Classroom and those on Gradebook but I'm also noticing that the first student in the class is repeated so I'm wondering if that might be why?