One issue is that Sturgeon truly believes that Scotland as an EU member state could hold on to the pound, while the EU has made it clear that all new member states are obligated to adopt the euro after a transitional period. Brussels has no real incentive to make an exception in the case of Scotland.
not strictly true, Denmark (and the exUK) have official opt outs, the Swedes have an official policy of intentionally failing to meet the criteria for joining and the Poles and others are also dragging their feet
to me the obvious route after independence is to keep the GBP for a year while setting up a mint and a central bank (or working out how scotland's note-issuing commercial banks operate from now) then bring in a separate Scottish pound pegged to GBP for a while before letting it float off, stability is maintained through the transition and the issue of adopting the Euro is kicked safely into the long grass for long enough to keep everyone happy