Here’s a corrected version of your sentence:
"Please refer to the attached proof of payment for Unitec. Please submit it to Unitec and request them to issue a payment receipt."
Changes made:
- Corrected "attched" to "attached."
- Changed "unitec" to "Unitec" for proper noun capitalization.
- Capitalized "PLease" to "Please."
- Changed "payment receipt" to "a payment receipt" for grammatical clarity.