An item is referencing a sales account: '[sales account]' which does not exist in Xero - Refer to Item: '[Item Name]'.

Error

The Sales Account used for the item/s in ConnectWise GL Mapping does not match or does not exist in Xero Chart of Accounts.

If the item mentioned on the error is a Product item (created in Product Catalog)

Search for the item and take note of its product category and subcategory.
Go to Systems > Setup Tables > GL Accounts and look for the Category and Subcategory mapped for the item.
Ensure that the Sales Account number is existing and exactly match in both ConnectWise Manage and Xero.
If the changes have been applied in ConnectWise GL Account mapping, the invoice GL Entries has to be refreshed.
Set the invoice status to GL Reload > set back to Closed or Closed - Emailed or for users with ConnectWise 2019.5 version and up, go to Finance > Accounting Interface > Unposted Invoices > select the invoice/s > click on Actions > Update GL Entries. However, if changes were only applied in Xero, you may proceed with re-syncing the invoice (Step. 5).
Re-sync

If the item mentioned on the error is a Service item (For example REV.SERVICE (this the NON-ProductID set in your GL Account Mapping)

Go to Systems > Setup Tables > GL Accounts and search for the account number or the item mentioned on the error.
Ensure that the Sales Account number is existing and exactly match in both ConnectWise Manage and Xero
If the changes have been applied in ConnectWise GL Account mapping, the invoice GL Entries has to be refreshed.
Set the invoice status to GL Reload > set back to Closed or Closed - Emailed or for users with ConnectWise 2019.5 version and up, go to Finance > Accounting Interface > Unposted Invoices > select the invoice/s > click on Actions > Update GL Entries. However, if changes were only applied in Xero, you may proceed with re-syncing the invoice (Step. 4).
Re-sync