Michael's mom asked him to pick up a few things at the grocery store. For taco dinner, beans, cheese, and salsa. Which should a: be placed to correct the error in the sentence?

There is no error in the sentence.

In the given sentence, the error is with the word "a:". The correct word to use in this context would be "should". Hence, the sentence should be corrected as -

"Michael's mom asked him to pick up a few things at the grocery store. For taco dinner, beans, cheese, and salsa should be placed."

In the given sentence, there is no error since it clearly states that Michael's mom asked him to pick up a few things at the grocery store for taco dinner, including beans, cheese, and salsa. Therefore, no correction is needed.