Review the original item:
My cat Buster is always tipping over the garbage can
then he searches through the trash for milk jug caps, which he bats
around the floor, the feline version of playing ice hockey.
You wanted to fix it this way:
My cat Buster is always tipping over the garbage can,
then he searches through the trash for milk jug caps, which he bats
around the floor, the feline version of playing ice hockey.
To add a comma between can and then would create an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. My cat Buster is always tipping over the garbage can is the first main clause. Then he searches through the trash for milk jug caps is the second main clause.
The spot between can and then needs a stronger break than just a comma.
You might want to consult the rules for fixing comma splices and fused sentences.