QB Issue Resolution:

You can click the + sign in the upper right corner and select EXPENSE under the Vendor menu.  That will bring up the screen to enter a single expense transaction.

 

Enter all details you want such as the offsetting account and in the top of the screen, select the AMEX account as the payment account.

 

(picture below)

Resolution for Issue 'How do I add one missing AMEX transaction?' available: Yes (Solved).
Source: Intuit Community forum.

How do I add one missing AMEX transaction?: 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 AMEX transaction
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Error Recovery for AMEX transaction [Canada]
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