People of any age can date each other. If either of them are under age their parents have to agree to the dating. It is never illegal to date with parental approval.

Can a 15 year old have a relationship with a 17 year old?

There is certainly no law against a 15 year old and an 17 year old arranging to meet and go somewhere together, or refering to each other as my boy/girlfriend. It is against the law for anyone to have sex with or to do sexual acts with anyone who is under 16.