It's great to know you're taking a QuickBooks business management class. Let me share some things with you, @Suzque.

The option to manually add the Undeposited Funds account when creating a Deposit is unavailable. You'll encounter the warning message since the Undeposited Funds is an internal software account. What we can do is to put the put payments into that account. Then, record a bank deposit and combine the payment in QuickBooks.

1. On the Homepage, select Record Deposits / Make Deposits.
2. In the Payments to Deposit window, select the payments you want to combine. Then select OK.
3. In the Make Deposits window, select the account you want to put the deposit into from the Deposit to dropdown.
4. Check the deposit total. Make sure the account and selected payments match the deposit slip from your bank. Use your deposit slip as a reference.
5. Enter the date you deposited at your bank.
6. Add a memo as needed.
7. When you're done, select Save & Close.

From there, all payments in the Undeposited Funds account will automatically appear in the Bank Deposit window. See Deposit payments into the Undeposited Funds account in QuickBooks Desktop article for the steps and details.  You can also check out the image below on how the account works:

For future reference, the following articles provide an overview of depositing payments in the Undeposited Funds account as well as instructions on how to record your account receivables.

• Deposit payments into the Undeposited Funds account
• Accounts Receivable workflow

I'm always here to help if you have any other concerns or questions about bank deposits. Just tag my name in the comment section and I'll get back to you as soon as I can.