The error is a Validation Error and is occurring due to the following validation rule:
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):
Tick the line to select it 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 the Rtn key. This should then update 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)
Tick the line to select it to be matched
Select the Match Partial Quantity check box.
Edit the Qty Matched to update the Total Matched. This will calculate as:
Sum of Qty Matched * GBP Price = Total Matched Value