Does it slip on one cog or all of them?
Usually:
One cog = a cassette/cog problem
All cogs = a chain problem
Slowly pedal the crank backwards with your hand. Look at the chain near the jockey wheels and look for a stiff link. There are other causes including a bent RD hanger but a stiff link is usually to blame.
Last edited by nomadmax; 10-19-19 at 04:31 AM.