If you have placed an order on the Round App and it was cancelled then your refund will be processed by Round not the Students' Union.
Your refund should be back in your account within 7 working days. However if you have further enquiries regarding this then you will need to contact Round direct.
You can contact Round at help@round.app
I'm afraid the Students' Union has no control over the refunds as payment is taken and processed by Round.