I know I've sent an invoice to my client and also been paid by them, I just can't find it in my quickbooks

Thank you for posting in the Community, @massimo.

Let's attempt to check a transaction report first to view the customer's invoices you've sent. If the invoice was paid through Payments, this should create a receive payment transaction deposited to the Undeposited Funds.

Once the payment is already in the Undeposited Funds, you'll need to check the Bank Deposit screen in QuickBooks and add the deposit to your actual business account. This is where you can locate the invoices paid by your clients. I've attached a screenshot for your reference. 

If you're ready to record and deposit this payment, you can check this guide for the outlined steps: Record and make Bank Deposits in QuickBooks Online.

Let me know if you have other questions. I'm here and ready to help. Have a good one.