The reason you see this error message is that the Tax Amount for the item is greater than the Net one.
The issue is that Xero won’t accept this via their API even though it is actually correct for the item.
The work around for this is to split out in two lines using the ‘Edit Line Items‘ feature and use the tax rate “VAT import” to apply 100% tax for one line and then the normal 20% on the other.
Here is a screenshot example how this may look like:
