Can I break a single speed chain with kmc universal link
It depends on the width of the chain. Single speed chains come in 1/8" and 3/32" widths, so you need to know whats on the bike to use an appropriate master link.

You should also check the chain, chainring and cog for wear. If the chain is old and poorly maintained it may be time for a new one (and possibly replacement of other drivetrain parts).