Review the original item:
Slouched in his seat, Michael tried to pay attention
the problem was that Nina's smile before class was distracting him
from Professor Tyson's grammar lecture.
You wanted to fix it this way:
Slouched in his seat, Michael tried to pay attention,
the problem was that Nina's smile before class was distracting him
from Professor Tyson's grammar lecture.
To add a comma between attention and the would create an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. Michael tried to pay attention is the first main clause. The problem was is the second main clause.
The spot between attention and the needs a stronger break than a wimpy comma.
You might want to consult the rules for fixing comma splices and fused sentences.