Overview
Within MV2, there is a page named ‘Log Viewer’ that will display a filterable list of messages that have been written to MV2’s log files. This article will focus on the 'Provider Errors' section of the Log Viewer, where issues related to MV2 Transactions failing to update Business Central will appear.
Accessing the Log Viewer
While logged in as a User who is assigned the System Administrator Role, from the Portal Menu, select Administration -> System Administration -> Log Viewer.

From the Log Viewer, select the 'PROVIDER ERRORS' tab. This tab contains any information about transaction data that was unsuccessfully sent back to Business Central. Filtering is available on Date Range. Clicking on a row will provide additional details about this message.

!!! Need more screenshots here of the deeper dive into the messages.
Link to the list of common errors returned by Business Central and their causes:
Troubleshooting - MV2 Transactions - Common errors returned by Business Central