When the account you choose for the transaction is Undeposited Funds, you will get this prompt message since this is working as designed. Undeposited Funds is the source account for the transactions listed in the "Select the payments included in this deposit" section. While the section "Add funds to this deposit" is for adding funds not contained in undeposited funds to the deposit slip.
This way, I'd suggest checking the account you choose for the transaction. If it's Undeposited Funds make sure to change it to another account so you can save it without any error. For additional details on how to properly record a bank deposit, click these articles:
However, if it's the correct account and you still get the error message, I'd suggest logging in to your account using the supported browser like Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge. This way, we can further investigate this issue. For more details check this article: Supported browsers in QuickBooks.