Can we say:
The country was in a turmoil
instead of:
The country was in turmoil
I have searched the web and found that it is used without "a". But why is it so? Is the first use incorrect?
Until the middle of the last century we usually included the article...
...but the current preference is now so strong that I think most (particularly, younger) people would probably notice and think it slightly "odd, dated" if you included it today.
Another closely-related usage where the same "article loss" has occurred over roughly the same time-frame is (to be) in (a) tumult. But it's interesting to note that although historically there never was an article in to be in (a) high dudgeon, you'll see from this chart that the article has started to appear there (but it's still far less common than the "standard" version).
I would say there's a general trend towards "simplification" in English, so "unnecessary" articles may sometimes disappear. With relatively uncommon words like turmoil, tumult, dudgeon there will always be some native speakers who simply aren't familiar with standard usage anyway, but on average if they're going to get it wrong they're more likely to err on the side of parsimony (if you don't know you need something, it's usually better to discard it than keep it).
In a turmoil trivializes turmoil.
This is going to sound silly - but it's because use of the indefinite article makes it seem like there's a group of "turmoils" just waiting to happen to countries that can be assigned at will.
You may want to do this for something minor or funny, e.g. "The country is in a turmoil over Trump dyeing his hair green."
This doesn't happen with in turmoil - because you are not specially addressing the question "which turmoil" with an article, it comes off stronger as a state that the entire country is in, rather than a "thing" it is experiencing.
As turmoil typically does not describe something trivial it's not surprising in a turmoil is not common.
Turmoil is a state or condition similar to chaos.
Things can be in turmoil or in chaos.
They can not be in a chaos or in a turmoil.
This is similar to other states or conditions such as love, ecstasy, heaven, distress.
People are in love not in a love.