Error message in Moodle Gradebook Course Total
In the Moodle Gradebook > View > Grader Report or User Report -- The word "Error" appears in the last column "Course Total". Or it might appear in a Category total.
There's a faulty formula in the manually-created Category or Course total calculation.
Correct (or delete) the faulty formula.
In the Grades > Gradebook Setup > Calculator (icon) ('Edit calculation for [category or item where "Error" appears]')
Correct or delete the formula.