Hi! I have a 7 speed mtb and recently i change the cranckset, since then i was not able to keep the chain on the lowest gear with the high chainring. I already shorten the chain with 5 links but no difference. What should i do?

Does the BB have the recommended axle length for the new crankset?

Indiscriminately shortening your chain could cause the RD to end up in the spokes of the rear wheel, and you in the Emergency Room at the local hospital. Set the chain using one of the suggested methods, I use big-big plus 1 or 1½" so as to not use a half link. Park Tool's website has a good description. https://www.parktool.com/blog/repair...-length-sizing

I have a square taped bb and don't see how a identical cranckset could make any difference, I can tell that is not the cranck fault because after it falls over the largest chainring on the second one, the chain falls from the second one to the smallest chainring too.
If the larger gear means the big cog they yes it can't work togheter

I only think that if the seller says it has 21 speeds it means 3 chainrings * 7 cogs=21 speeds