Review the original item:
Running through the parking lot, Aisha hoped to get to her car before the
rain began when she found the doors locked
and saw her keys lying on the front seat, she knew that she was in for a
soaking.
You wanted to fix it this way:
Running through the parking lot, Aisha hoped to get to her car before the
rain began,
when she found the doors locked and saw her keys lying on the front
seat, she knew that she was in for a soaking.
To add a comma between began and when would cause an equally bad problem, a comma splice. A comma splice occurs when you have two complete sentences joined with a comma alone. Aisha hoped to get to her car ... begins the first complete sentence. When she found the doors locked and saw her keys lying on the front seat, she knew ... starts the second complete sentence.
The spot between began and when needs a stronger break than a wimpy comma.
You might want to consult the rules for fixing comma splices and fused sentences.