The errormessage is quite clear only discounts pcts between 0 and 100 are allowed and yours is negative. So the problem is your Unit Price/Line Amount.
If you cannot see what is wrong with the specific line yourself, then let us see the line so we can guide you.
Unit Price is zero on the screen provided - the Service Invoice Line is created as a funny mix of SRV-0001 and the Service Contract Line - so you must have a bad Unit Price somewhere.