I fixed it! - IN SHORT: REPLACE THE HEATER AT THE BACK. IT’S EASY!
I had the same problem recently.
Whenever you search for it, it shows the error code E21 result of another dryer.
I even asked an expert from a particular website and they suggested it means water left in condenser or condenser pump not responding. - NOPE this is again the same as the google search I conducted myself which was useless.
I figured that if the heater isn’t working I’d have a look at it. Took off the screws at the back around the centre surround and then two screws for the heater and pull the plug out which is behind the rubber where the wire goes into the dryer.
Replace with replacement heater and it works.
USE A PROPER GUIDE
I got mine from RansomSpares £34.95 part number RS741100