First i would try lube under the belt.
Use 100% liquid silicone
Start at the front, peel the edge of the belt up and spray under as far as you can, working your way to the back of the machine. Repeat on the other side.
Then walk on the machine for a few minutes to spread it evenly around.
If it is still slipping, tighten the rear roller bolts 1 turn each. (righty tighty)
If it is still slipping you will need to take the motor cover off the front of the machine and inspect the drive belt.
If the belt looks ok and there is not a lot of debris (black looking dust) then you can try to tighten the belt (if possible)
You do this by loosening the motor itself, and depending on the model, move the motor farther away from the walking belt. Then tighten the motor and try it.
If you still have a slipping problem, then that leaves a bad front roller.
The pulley has broken loose from the roller and you need to replace that entire roller with pulley.
If you do need to replace the roller, i would replace the drive belt as well, they are cheap and you need to remove it to install a new roller, so its a good idea to replace it.
If you need more assistance, just reply back, if not please rate my answer, thanks!