"To update fields on a paid invoice line item, you must supply a LineItemID" Xero error

"To update fields on a paid invoice line item, you must supply a LineItemID" Xero error

This error means that Xero is either missing a Product Code or not able to access or edit it when processing the posted order.  Possible things to check:

1. Connection to Xero dropped - If so reauthorize the connection https://support.xeroom.com/support/solutions/articles/77000534945-xero-connection-broken-type-null-title-unauthorized-status-forbidden401-detail-authenti

2. Missing or Invalid SKU codes in WooCommerce - Xeroom will take the product ID as a fall back if no SKU is supplied. Sometimes this might not work in the case of variable products.  Also check for use of invalid special characters in product codes eg / \ ? etc.

3. Voided Product Codes in Xero - Check that the codes you are using in the order are all valid in Xero and not voided.

4. Invoice No, Most Likely Cause - If you are fixing the invoice start number it can lead to this error.  If you have existing invoices with the same number sequence then Xero gets confused as it tries to and fails to update an older or voided invoice that might also be in an ealier closed accounting period.  The solution is to relax this constraint and leave it blank so Xero can create a new invoice as intended instead of accessing an existing one. If you wish to set a specific start point for your Xero invoice numbering then ensure that it is higher than all of the historic invoices that you have in Xero.