Creating Alerts for Non-matches

I wonder if it is better to highlight the "bad" choices instead of showing separate alerts, in case there are multiple "bad" choices, it is easier to highlight than to list them

See attached.

First create a listing of all courses and their corresponding prerequisites (if there are no prerequisites, then list the course and relist it as its own prerequisite).

Then apply conditional formatting by selecting entire range C2:F3 and go to the Home menu and selecting Conditional Formatting >> New Rule >> Use a formula to determine which cells to format.. then apply formula:

=ISNA(MATCH(INDEX($M$2:$M$7,MATCH(C2,$L$2:$L$7,0)),$C2:$F2,0))

Click Format and choose from the Fill tab.

Of course, you would edit the ranges to suit where you put your lookup table and your input fields.