While starting with saying you still haven't told us which tree you're trying to grow, simply 'cause most of the trees used for bonsai can't/won't survive indoor.

After all, that's not where they evolved to grow, and it's a very hostile environment for most plants.
Only exception is tropical plants (like ficus), for which the environment is still suboptimal (they'd need very high humidity levels, and there's no doubt you can't provide that in your house), but less than temperatures getting too cold: those can be bonsai, but hardly if kept in an apartment, as they'll hardly get strong enough. They'd need a greenhouse of some sort to do well enough, in general.

umm wrote:

Why is that?

Trees die inside. There is no such thing as an indoor tree. If you check the help section here on this forum you will see that 99% of the "help my tree is dying" questions are from people with socalled indoor trees. Some tropical species can stand the lethal environment that our living rooms are for them for some time, but they will do much better placed outside when temps are suitable for them.
Most/all trees from temperate climates need a dormant period ( winter) , without that ( being inside with the same temperature and dry environment) they just die.

Oh sorry, I’m growing a dwarf jade

umm wrote:

Oh sorry, I’m growing a dwarf jade

You said you had a juniper?

From what I’ve researched it isn’t ideal to grow a tree inside but you can grow some subtropical and tropical trees indoors. I’ve had the tree for about 2 months and besides a small light deficit the tree has been ok. I keep it outside when it it’s not getting enough light so for the most part it’s been in good health. No signs of dying or weakening.

I do, I am growing that outside.