"I have a high temperature."

Translation: Я маю високу температуру.

[Angeparmar]

Why can't i say у мене є висока температура?

[Alexis-Mario96]

Difference between "у мене є" and "у мене"?

[Tielbert]

У мене є is I have smth, I own smth, e.g. bag or dignity. You can hardly say that you own high temperature :) Though, the boundary between these two is rather blurred.
To tell the truth, У мене є висока температура is not quite wrong, but sounds weird.

[Pawel73418]

In Polish we own high temperature ("mam temperaturę" like "mam samochód").

[Tielbert]

In Ukrainian you also may say "я маю температуру".

[Pranon]

Why is it "високу" instead of "висока"?

[jmango2]

Because there's not a direct object of a verb in there. У мене висока температура is literally "there is a high temperature with me", which means high temperature is the subject and has to be in the nominative.

If you used the verb мати, then you would have a direct object and need the accusative form: Я маю високу температуру.