Hey, @will_t024! We'd recommend trying the troubleshooting steps on our support page: https://www.samsung.com/uk/support/repair/ 😊
I've had the same problem with my S20 plus, I don't care what anyone says, it's a software issue if your phone has been kept dry!
The way to remove the constant warning & get it charging again is to plug it into a running laptop/pc & the warning will go & then you can plug it into your proper charger. Mine hasn't done it for a while until yesterday when I bought a USB c card reader with OTG built in & the moment I stuck it into the phone... Boom, moisture detected! So it's 100 % a software problem.