Nobody still knows what customs arrangements we will have if / when brexit occurs, despite blundering brexiteers saying negotiations would be a doddle, when they actually tried they did sweet FA for ages then ran away frit (stares at D Davis)
Even if there had been competent negotiations, given track record of gov IT projects (ranging from total clustur**** to merely shambolic) then anyone with a room temperature IQ would have bet money on HMRC new IT systems running late / or bodged on time release of unfinished product (and so having major showstopper bugs)
Theres blame in HMRC IT team, but also blame in the whole brevity timetable which was way too fast - anyone with a clue would have devised contingency plans for various outcomes, ensured systems were in place and then declared A50.
Instead the jingoistic racists* just wanted to exit ASAP.
* and the non racist easily deluded s who believed the lies about a fantasy world of better economic prospects outside the EU, a well funded NHS (hint, lots of high up MPs are cozy with companies that would benefit from privatized NHS, only way they will pump in lots of cash is if said cash mainly goes to their business mates rather than on real patient needs)