LTI and Moodle

Hello All, 

I am a newly registered user who has found many great resources in this forum. Thanks to all!

I just set up a Moodle 3.9 LTI provider and a Moodle 3.9 LTI Consumer. I set up an auto-graded quiz and LTI connection to the consumer site. 

Where this is at:

1. The activity-level LTI tool works (it is not a course LTI)
2. I have a non-editing teacher account in a group with a student account
3. The teacher can view attempts and see the grades in the Quiz LTI Iframe loading on the consumer site
4. The Provider LTI is set with Grade Synchronization turned to YES
5. The Consumer LTI tool is set to "Accept Grades"
6. Cron is running on \enrol_lti\task\sync_grades the LTI Provider and the LTI Consumer with 0 writes on the Provider and 0 reads/writes on the consumer. Here is the task log output on the Provider site for the LTI tool I am testing:

Starting - Grade sync for shared tool '625' for the course '14'.
Skipping - Invalid grade for the user '7' in the tool '625' for the course '14'.
Failed - The grade '1' for the user '21' in the tool '625' for the course '14' failed to send.
Skipping - Invalid grade for the user '4' in the tool '625' for the course '14'.
Completed - Synced grades for tool '625' in the course '14'. Processed 3 users; sent 0 grades.

I assume the Skipping - Invalid grade refers to the users enrolled in the course who have not taken the quiz (such as Admin)

This line is the one I am curious about: 
Failed - The grade '1' for the user '21' in the tool '625' for the course '14' failed to send.

7. The Consumer side Gradebook for the course in which I have placed this tool shows the Quiz in the table with just "-" as the grade entry for the student account that completed the quiz.

I have read through several discussions in this forum and have still not been able to fix this issue. I appreciate your suggestions! Thank you.