Data check/validation when inputting?

I am trying to create a spreadsheet that shows trainers availability to deliver courses.

In Sheet 2 I have a character 'n' next to a date when a trainer is not available.

In sheet 1, when inputting trainers names next to a date to forecast future courses, I would like to generate or flag/error message that a particular trainer is not available based on the data in sheet 2.

Anyone help?