This error occurs because the teacher data is not properly populated in the Teacher table (elementary) or the section (secondary) for multiple teachers.
Use a query to look at the multiple teacher field values for the class (elementary) or section (secondary) indicated in this error. If the section/class has multiple teachers, then confirm the Staff ID and SEID are populated for each teacher record linked to this section/class. If the indicated section is NOT being taught by multiple teachers, determine which fields need to be deleted and updated. The following queries may be helpful:
LIST TCH TE MTS ID2 ID3
LIST MST TN MTS TN2 TN3