i need a refurbished macbook air 2015 that I can pay on installment

Any of the refurbished units from Apple can be purchased with Financing (installments) from Barclaycard. See here: https://www.apple.com/shop/browse/financing

https://www.apple.com/shop/product/FQD32LL/A/Refurbished-133-inch-MacBook-Air-18GHz-dual-core-Intel-Core-i5

Beyond that, your own credit card company may offer the possibility of installments on any purchases you make with it.

That's what credit cards are for. Go to Apple.com. Scroll to the bottom of the page. You will find a link to the refurbished and clearance store there.