Correct version of "Space Bar"

Definitely not capitalized: a Space Bar would be something like a Mars Bar or Milky Way, but less dense (facetious confectionery reference).

But whether you include a space is up to you. Originally so: the space bar is just below the X key, which requires a space to avoid confusion. However, spacebar is common enough not to be ambiguous by now.

According to the 4th edition of the Microsoft Manual of Style, Spacebar should be capitalized, similar to the other keys on the keyboard.

Several general language dictionaries reference the word without capitalization, written as two words (cf. Oxford Learner's Dictionary, Merriam Webster).

To make matters more confusing, Microsoft's Terminology Portal proposes several different variants: SPACEBAR, Spacebar, and spacebar.

A search in Google Books provides yet more examples illustrating why we're all asking ourselves this question; SPACEBAR, Spacebar, space bar, and spacebar are all found in publication.

Good luck!