You can write anytime as one word or as two words when you are using it as an adverb.
I am available anytime, if you’d like me to help you with your novel.
I am available any time, if you’d like me to help you with your novel.
If you’re not sure whether you’re using anytime as an adverb, try substituting some other adverb like “quickly” or “loudly.” My new bicycle allows me to go anywhere quickly. If the adverb works, then it’s OK to make anytime one word. You can also make it two words, but to some readers it will look old-fashioned or more formal.
You can also use anytime (or any time) as a conjunction.
Anytime the sun went down, we’d go for a night-swim.
Any time the sun went down, we’d go for a night-swim.
There’s no difference in meaning.