As BOAC has indicated, the question cannot be properly addressed, but as a generic answer, YES, the message must first be cleared before the scratch pad information may be inserted with the LSK.
Looked at in simple terms, the LSK accepts the information in the scratch pad data field, and, to enter the desired information from that field, the "Surface Message" must first be cleared - what is on display is what the LSK attempts to enter. Your data input should still be there after clearing the message. The programming logic requires that a "blind" entry cannot be made, i.e. there can be no 'taking on faith' information, however correct, is hidden behind other data.
This one can be a real pain in the butt when entering flight plan winds for a long flight, early in the flight strong Headwinds followed by later much more favourable winds, forward wind propogation "assumes" continuation of the already entered Headwinds for the entire flight, giving a repetitive "INSUFFICIENT FUEL" scratch pad message whilst you're trying to continue entering the later winds.
Regards,
Old Smokey