Hello Kiril. The logline is good. Two things though, you can remove the 'to' after 'witness', and I don't really think the three dots after 'accident' and in the end of the sentence are necessary.

You have the start of a scary movie, Kiril Maksimoski.

Here are two loglines (the first is one sentence, and the second is two sentences):

"After a young couple witnesses a devastating car accident while on vacation, they ________ (goal of story) so ________ (stakes)."

"While vacationing in a stranded cottage, a young couple witnesses a devastating car accident. ________ (goal of story) so ________ (stakes)."

Suspenseful, spine-chilling. Hopefully, you can make the screenplay available in the future.