What can I do about my institution taking a long time to correct a mistake in my transcript?
Wait

You should do nothing. The universities made a mistake and claim to be fixing it. Unless you have a reason to believe they will not fix it, then you should do nothing but wait.

Mistakes happen and it is unfortunate.

If you have a reason to believe the universities are not honest and will not fix the issue, then you should edit that reason into the question.