This is a validation error. The rule requires:
If Total Matched - Invoice Total (Outstanding to Match) = 0 THEN Validation Pass ELSE Validation Error
What this is essentially saying is that on the screen, does the Total Matched figure, minus the Invoice Total equal to 0? OR is there still an Outstanding to Match value.
If this is true (remaining Outstanding to Match) and all the lines you've selected add up to the Invoice Total, then this may be down to the Order Lines Qty Ordered.
If you spot a line with a Qty Ordered >1 then you'll need to:
If matching the entire line (i.e. the whole value of the line to be matched to all or part of the Invoice Total):
Select the line to be matched.
Double-click on the GBP Price and re-enter the value (for example, if this was 1.00, re-enter 1.00), then press Return. This updates the Total Matched.
If partially matching a line (i.e. a fraction of the value of the line to be matched to all or part of the Invoice Total)
Select the line to be matched.
Select the Match Partial Quantity checkbox.
Edit the Qty Matched to update the Total Matched. This calculates as: Sum of Qty Matched × GBP Price = Total Matched Value.