Arsthbt

I am going to watch Avengers: Endgame soon but I have not seen Ant-Man and the Wasp and Captain Marvel yet so I wanted to know if I can skip them and still have no issue to understand Avengers: Endgame or not?

Both movies are set before Infinity War, so it's really not that important to watch them. Although, I'd say you have to watch mid-credit scenes of these movies because they play as teasers for Endgame.

You may skip Captain Marvel though if you have background information about that character. However, you won't know the connection between Captain Marvel and Nick Fury, but it's not really important.

Ant-Man and the Wasp is also set before and during Infinity War. The only thing you need to know is the Quantum Realm which plays an important role. However, you can still get an idea about it while watching Endgame.

From Express article,

While both are set before Infinity War, they could contain something of a teaser for what the Avengers’ plan in Endgame will be in the fightback against Thanos.

But ultimately, if fans don’t have time to see either movie before Endgame it isn’t the end of the world.

However, both films’ mid-credits scenes are the only thing that audiences really should watch before the Marvel epic is out this weekend.

I would like to explain these two mid-credits scenes in addition to AJ's answer.

Ant-Man and the Wasp

In a mid-credits scene, Pym, Lang(Ant-man), Hope, and Janet plan to harvest
quantum energy to help Ava remain stable. While Lang is in the quantum
realm doing this, the other three disintegrate.

Note: You might not know these characters, but actually you just need to know is that Ant-man is been trapped in the quantum realm.

Captain Marvel

In a mid-credits scene, set in the present day, the activated pager
is being monitored by the Avengers when Danvers appears.

That's all you need to know before watching Avengers: Endgame.