QB Issue Resolution:

Directly in the Receive Payment screen, you see the field for you to enter their Check #, to the right of Payment Method is Reference # or Check #, such as “EFT” or “ACH” or “1201.” Below this is a Big Box labeled Memo, for you to put in Notes about this Payment.

Resolution for Issue 'Adding custom internal notes to an invoice.' available: Yes (Solved).
Source: Intuit Community forum.

Adding custom internal notes to an invoice.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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