Thanks for this detail Lori,
As the error message states, there could be a number of reasons for this issue.
The first thing I would test is changing the Client Name IN QBO (I usually simply add "QBO" to the end), and see if the client name updates back in ServiceM8 on the same Job (may take some time). This should rule out an issue with the Client record.
Next I would verify that all Line Items on this Job are formatted correctly; Quantities, Pricing, etc are correct with no oddities, the descriptions do not have odd or special characters, and all line items have Item Numbers. If all is well here, are all the Item Numbers already existing Materials in your account (are any of the Line Items "New" to this Job and would ultimately create new Materials in your account)?
I might be worth
scheduling a support session with our team so we can walk through these items with you in a recorded screen-sharing session and document the issues further ($40/instance). We can then connect and share this as well with ServiceM8 Support to further assist in their troubleshooting.