I'm having this same issue.  Can't apply the credit to an open invoice.  I've read the thread, attempted the sequence, cleared my cache, tried three different browsers.  Unsuccessful.  Following these instructions:

https://quickbooks.intuit.com/learn-support/en-us/help-article/customer-refunds-credits/create-apply...

In item #7 making the 'amount received' field blank, QB removed the ticks for the outstanding invoice and the credit.  Very frustrating.