Your bike is probaly older than the 'Lad' whose testing it. Its a crappy 30 year old 6v systam, so wat if a light dims when you push another button, next year take it somewhere else.
As for this year, make sure the battery electrolyte is topped up bwith de-ionised water, fully charge it overnight, check and clean any grounds you can find. As for ther horn, take a jumper direct from battery positive unplug the wire thats is NOT black and touch the jumper on the horn terminal.
If it works you need to strip down the horn push and clean the contacts
If it doesnt you need a new horn, of a new battery, or charge the battery etc
my money will be on cleaning the horn push contacts.