How do i remove this space in my union?

There is always this space in my unions and its quite annoying that my avatar floats instead of being on the floor of the union

Such is a result of collision accuracy in unions with concave volumes, could it not be made of parts instead? My recommendation would be to disable its collisions and remake the hitbox with invisible parts, but from the screenshot it’s planar enough it looks doable with parts to begin with.

thats just the way unions work so try building that out of parts.

It’s a problem with Unions and Meshes, especially when you are dealing with large ones.
Here’s a easy to use Plugin by @Maximum_ADHD that helps to see how they are rendered physically rather than visually:

Other fixes are to make your Unions/Meshes smaller, change the Union.CollisionFidelity property from Default to PreciseConvexDecomposition, build from Parts as @dizue said or just make transparent parts as @MP3Face said.

This plugin is new to me, so can you tell me how it works please?

There are directions in the description in the link I showed you.

Basically it turns on and off the “ShowDecompositionGeometry” tool in Studio Settings. You can manually do this, but it takes about 5 steps to make it work. The tool allows you to do it in 1 click.

The thing with Unions is that collision system aint meant to be 100% precise to prevent performance issues related with collisions, if you wanna have precise collisions, use PreciseConvexDecomposition in CollisionFidelity property, this will fix collisions, but be careful it might heavily impact perfrormance, also, I suggest you use roblox CSG only when there is real need in using it.
And as many others suggest, get the Mesh Optimization Tools plugin, it will allow you to see the real collisions instead of the visual Union.
(I can be theoretically incorrect, but if every Union would use PreciseConvexDecomposition, this would lead to performance issues if overdone)