You are certainly allowed to ask for exclusive rights to the artwork, although it is unnecessary to do so.
Upwork's default contract is VERY clear. Clients obtain all rights to the work that they pay for.
Unless you and the freelancer create a separate additional contract or agreement, then it is Upwork's default contract that is in effect.
Are you allowed to sell prints of the artwork?
Do you need to include the artist's name or give the artist credit?