ThereAs @user indicated, there is very little preventing the EU27 and the UK to negotiate a trade agreement, to be ratified at the same time as the withdrawal agreement. The main constraint is that such a trade agreement could hardly take effect before Brexit because it would probably entail leaving the customs union and single market (although even that would depend on its exact contents). In fact, the UK and the EU have already entered negotiations on this topic (albeit with no intention of wrapping them up by March 2019).
But that's actually moot, “take account” is very weak wording, that's the kind of things you end up with when some parties want something to be acknowledged but others do not want to seriously commit in any way. If the redactors of the treaty had agreed that the future relationship should be covered by the withdrawal agreement, the article would simply read “setting out the arrangements for its withdrawal and its future relationship with the EU”.
As it happens, the UK wished to discuss it as much as possible from day one of the negotiations and the EU27 did not. I would be curious to know if there was a similar disagreement when drafting the article.