The Scotland Act 1998 contains a provision that the Scottish Parliament cannot do any act inconsistent with EU law. Under what is still called the Sewel Convention (even though Sewel himself is only slowly recovering his reputation since he was photographed wearing an orange bra and snorting cocaine in the company of a prostitute) and is now S.28 (8) of the 1998 Act:

'it is recognised that the Parliament of the United Kingdom will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament.'

Withdrawing from the EU would affect 'devolved matters, thus any legislation to do so would (normally) require the Scottish Parliament's consent. First Minister Nicola Sturgeon said in very clear terms that she would encourage it not to give this consent, and indeed it seems inconceivable that Holyrood would do so if asked. Accordingly, runs the argument (if I have grasped it correctly), the UK Parliament could not lawfully pass the legislation that would be necessary for it to leave the EU.