Firstly, thank you reading and reaching out!
I will check this when I get a chance on Sunday but just from looking I would guess it may be a problem with the quotation marks. If you try deleting the “ ” characters (which you can see even here that Medium formats to make more pretty). Retype them in GraphDB and it should work without a problem.
The best resource regarding <urn> I can find is here.
The most up-to-date documentation (which explains an alternative to urn) is here.