Sounds like your condensate drain may be blocked..... Takes the condensation from the back plate of the fridge out to a reservoir above the motor to evaporate off.
If the small drain in the collector tray at the bottom of the fridge is blocked (usually with food products!), the resultant condensation will collect in the bottom of the fridge...
Been there, done that, got the tee-shirt.....
Bought a brand new fridge for an elderly neighbour years ago who had the same problem within a few weeks.... On inspection, discovered that the hole in the bottom of the collection tray hadn't been punched out at the manufacturing stage!!! Same result though....
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