Mk2s problem with slopping bed, how to fix

I have converted my prusa mk2s to use Duet 2 ethernet board.

I have created a height map in the web interface so I can use Automatic Bed leveling.
However the maps shows that the bed slopping from the front to back. Hence the back is higher than the front.

Does anyone know why this could be?

You are talking about an old Prusa which was/is made with threaded rod all over the frame? I would guess that your Z towers are not square to the Y axis. Or the threaded rod base and the bearinged bed rods are not parallel.