Question

correct the one error.
when are you coming back? i could use some help preparing for the party . . .
i totally forgot that all of your cousins is coming, too. so, i had to go back to the store!
i totally forgot that all of your cousins is coming, too.

Explanation:

The error is in subject - verb agreement. The subject "all of your cousins" is plural, so the verb "is" should be changed to "are".

Answer:

I totally forgot that all of your cousins are coming, too.