Check the movement fsm and narrow down which action is overriding velocity Y, you can just disable actions to see which. Probably set velocity. You want to set velocity Y to
none, because 0 means that the rigidbody will not move up (positive values) or down (negative values), hence it’s locked in place vertically.
When you hover over the the action, you should see tooltips. Playmaker actions are C# scripts that typically do one particular thing, as determined by the engine. That means, you can also look up what Unity documentation says how to use it, or use all those scripting tutorials on Youtube (you can always ask of course)