Handbrake warnjng light comes on when driving

The switch that detects when the handbrake is pulled is either loose and jiggling around or the handbrake cable is loose and allowing it to move enough to hit that switch. That's not safe, you need to have it looked at.

With a normal handbrake, you can remove some screws around the handbrake covering. The screws are sometimes well hidden so you have to look around.
When you get the covering off, there are two adjusting nuts at the rear of the handbrake. Turn the nuts until you can get "5 clicks" on the handbrake to set the brakes.
Some jack up the car while doing this so they can rotate the wheels by hand to feel when the brakes touch the pads.
Hope this helps

Sounds like the switch located on the hand brake arm that turns the "light" on out of adjustment, or as some do...it is failing. The switches are normally cheap.

We don't have this model here, but if the handbrake lever is in between the front seats, a common cause for this problem would be a coin that has dropped into this area where the switch that lights up the warning lamp on the dash is. As the car goes over bumps, the coin may move creating a circuit. This area would be the starting point for solving this problem.