Refusal to exchange good with proof of purchase

Needed to exchange goods bought from adidas strubens valley. Brought my proof of purchae as stated. On arrival I was told that they could not exchange the goods as it did not have the tag on it. Terms and conditions on the till slip does not state that the TVs must accompany goods. Manager was rude and more concerned about the corona virus. You cannot fit clothing on in store. So can adidas please explain how do customers now if the size they bought is correct.