I tried to redeem an iTunes gift card and it said that my card has not been properly activated

Could you ask the person who gave it you for the receipt ? If not you could try contacting iTInes Support and see if they can/will help.

This doesn't work because I got my iTunes gift card as a gift therefore not only do I not know what retailer it was purchased from, but I also don't have a receipt for the purchase. So how am I supposed to fix it now?

What you normally should do when you have an iTunes card that has not been activated is go back the retailer from whom you purchased the card and discuss the problem with them. They should either correctly activate the card or give you a new one. If they refuse to help, as some retailers do, or you don't know or can't get to the retailer, go here:

iTunes Support

and follow the instructions to report the issue to the iTunes Store. They will ask for a scan of the front and back sides of the card as well as a scan of the receipt

Regards.

Not help

What does 'not help' ? You've been back to the store that you bought it from and asked them to activate to the card properly ? If the can't / won't help contact iTunes Support, they will want images of the front and back of the card and its receipt : If you can't redeem your iTunes or Apple Music Gift Card or content code - Apple Support

i did that but it happened to the next one i got :(