Game starts, mouse cursor moves, but can't click & use keyboard.

Are you playing on Linux? Have you started the game through Steam?

Be sure to launch it from Steam, not standalone. It actually helps solve an issue too.

If on Linux this is a know issue but we fixed it with a trick when the game launches. It could be
that it doesn't work on your distro.

But try launching the game through steam and let us know how it goes.

You can also try editing playerprefs and play the game in windowed mode, this can very well solve the issue. But downside is that it is windowed.

open file ~/.config/unity3d/Bugbyte/Battlevoid_ Sector Siege/prefs

And edit

... is fullscreen mode type int... Set it to 0.