First off ask the original owner. 2nd double check the PUK for alternatives such as number 0 zero / letter O.
If you enter the serial number on this page (Victron Energy Support) it should connect you with the dealer who may be able to confirm PUK. They may ask for proof that you have the device. You may have to end up raising a support ticket if you get no luck.
On the support page use the lower enter serial number button in the location shown in the image below.