Is it check in or check-in?
If it is check in or check-in depends on the context.
If you’re using check in as a phrasal verb, then there is no hyphen (e.g., “We can check in after 3 p.m.”)
If you’re using it as a noun or as part of a compound noun, then hyphenate it (e.g., “Check-in is from 3 p.m.,” “The check-in time is 3 p.m.”).
QuillBot’s free Grammar Checker will help you use hyphens correctly in your writing.