We would never say simply, "I am learning a grammar."
The word "grammar" *can* be used to mean "a textbook that explains the grammatical rules of a language," and in this case you could say, for example, "I am studying a Latin grammar" or "Yesterday I bought an English grammar." You still would not say, "I am learning an English grammar," since you do not *learn* a book but rather *study* or *read* one.
But we can say, "I am learning grammar," often with an adjective in front of the word "grammar," as in the sentence "I am learning English grammar."
We can also say, for example, "I am learning the grammar of English," which means the same thing.