A doubt

Why isn't this sentence wrong?
"Imagine there's no countries" This is wrong.  It should be, "Imagine there's no country"

Shouldn´t it be "Imagine there ARE no countries"? Yes, this is correct.

After "no" the noun should be singular.  As you may known, this line comes from the song of John Lenon's Imagine. This how he wrote his song even though the English is bad in this sense.  His other line, "imagine no possessions" is also wrong.  This shows that even native speakers are not immune to perfect grammar.  This is his song and he can write it anyway he wants.

Lyricists of today are not what they are used to be before. So if you are famous, you can write whatever you want, whether right or wrong, the fans will swallow everything.  This doesn't make them correct.  So this is a caveat when you learn English from songs and movies...  Just make sure they are of quality.  As for his songs, he is a better song writer than a grammarian.

There is no problem that God cannot fix.
 
Thank you very much.

A doubt

Why isn't this sentence wrong?
"Imagine there's no countries"

Shouldn´t it be "Imagine there ARE no countries"?

Thank you very much.

They're song lyrics, laws of grammar may be turfed out the window for scan or cadence, for rhythm or rhyme.  'There's no country' would have worked too, and left no room for doubt.