You cannot add a billable expense to a paid invoice. As a workaround, you can create a separate invoice and add your billable expense, or delete and recreate the invoice

You may turn on billable expense once you have upgraded Your Quickbooks version here’s how:

https://community.intuit.com/articles/1145654

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Resolution for 'adjust invoices issue: 'Can I adjust invoices that I have received payment for to be billable expenses after upgrading to QB Plus?' available: Yes.

The adjust invoices 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 adjust invoices issue yourself by looking for a resolution described in the article. If it is a complex adjust invoices issue or you are unable to solve the issue, you may contact us by clicking here or by using other adjust invoices support options.

For more information about resolving issues related to adjust invoices in Quickbooks/QBO or to get support for the issue 'Can I adjust invoices that I have received payment for to be billable expenses after upgrading to QB Plus?', click the link below: