"Я не бачу кетчуп на столі."

Translation: I do not see ketchup on the table.

[SlickJimmy]

Is there any reason it can't be "the ketchup"?

[deactivated user]

No, it should be accepted, too. The Ukrainian sentence can be used both for an undefined quantity of ketchup of for a known pouch of ketchup.

[NazarBodan]

Still not accepted.

[arihuta]

Why is "I don't see a ketchup on the table" wrong?

[Cogitos]

Ketchup is an uncountable noun, so it can't have an indefinite article.