May I know whether the sentence below is grammatical?
" Please request for receipt if it doesn't reach you'?"
No, the sentence is not grammatical. Since you haven't said what "it" refers to, I don't know what the sentence means. I'll pretend the sentence doesn't include the "if it doesn't reach you" part.
One problem is that the sentence isn't a question, so your punctuation doesn't make sense. But the main problem is the use of "request for receipt," which doesn't work. You can change the beginning part of the sentence like this:
- Please request a receipt.
- Please ask for a receipt.