onGround Doesn't work/exist for me

[BubbaNathan]

I have been trying my hardest to find the method to detect if the player is on the ground. I looked it up and people just say it like onGround always works, or isOnGround always works. Nothing shows up for me and the wording is red. I tried looking up an import but it doesn't seem to exist.
I had to same issue with hasGlint, which turned out to be in the Item class and was isFoil instead. I even found the checkfalldamage method in the Living Entity class (I found this through looking at isFallFlying method and looking at usages) but it  uses its own variable to see if the player is on the ground, and I have NO idea where it gets that from!
(It is called pOnGround)

I am using Intellij, SDk 21, Language level 17, and the Forge version is 1.20.1-47.1.0. I Don't know where to look anymore and I feel like im going crazy.

[Choonster]