QB Issue Resolution:
I can understand that being confusing.
The Simple Start version of QuickBooks Online does not allow adding custom fields. That is available in the Essentials and Plus versions.
You would need to upgrade to Essentials to be able to add custom fields.
Resolution for Issue 'I want to add check numbers to my customers payments.' available: Yes (Solved).
Source: Intuit Community forum.
I want to add check numbers to my customers payments.: 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.Support for Customers
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