The Brexit negotiations with the EU have a circle that needs squaring - it's the border between the Republic of Ireland and Northern Ireland. The Republic want the border to stay completely open because they depend heavily on trade with the North.
The EU says if the UK leaves the customs union this can only happen if the North has a different legal status from the rest of the UK, ie remains quasi inside the EU. The DUP, who hold the balance of power in parliament, say no way - the North must remain identical to the other parts of the UK.
The UK would be willing to stay in the customs union except that the EU says if you're in the customs union you must allow free immigration. But the whole point of Brexit was to end mass immigration. There is no reason, of course, why a customs union requires free immigration but the EU won't allow one without the other; basically to coerce countries into opening their doors to the masses. They have already done it to Norway and Switzerland.
There is no real answer to this question, so, someone is going to have to crack.