A: It is a story about the Spring and Autumn pPeriods in China, when China was divided into seven countries, and then young boy who became the first Qin Emperor and Shin who young boy,and later became a generals later who unified China bythrough wars.

I think they will be able to unify one of them by the 20th year, so, by this calculation, it will take another 100 years to complete the series.

へー！

A: It i's a story aboutset in the Spring and Autumn period in China, when China was divided into seven countries, and thenbout first Qin Emperor and Shin who were young boy,s, and then became generals later and unified China bythrough wars.

I changed "It is" to "it's", because "It is" can sound formal or stiff, especially when it's being used in a dialogue.

This character says the story is about the Spring and Autumn period in China, but then talks about the emperors, so I made some minor edits to make that more clear.

A: It is very interesting.

"It is" sounds natural here because it adds a sarcastic tone to the dialogue.

But there i's one problem.

"There's" sounds more natural here, again because it's being used in a dialogue.

A: It is a story about the Spring and Autumn period in China, when China was divided into seven countries, and then first Qin Emperor and Shin whose young boy, became a generals who later unified China by wars.

A: ItThe story moves very slowly.

In short, they have to win over countries one by one and unify them one by one, but right now the series has been running for 17 years and about 70 volumes have been published, but not even onetwo countries hasve been unified yet.

I think this is what you meant

What i's the story about?

You could also say "No, what's it about?" to feel less formal

A: It is a story about the Spring and Autumn period in China, when China was divided into seven countries, and how then first Qin Emperor and Shin, who was a young boy, became generals laterand unified China byvia wars.

A: It is very interesting.

Grammatically correct, but just noting that when it comes to style most English speakers would consider sentences that don't use contractions to be more affirmative. Because the sentence stops, if I were to ask "Have you ever been to Japan?" a response of "I have not." feels more assertive than "No, I haven't." In this case, I'd say you used the perfect variant.

But there i's one problem.

In short, they have to win over countries one by one and unify them one by one, but right now the series has been running for 17 years and about 70 volumes have been published, but not even one has been unified yet.

I think they wi'll be able to unify one of them by the 20th year, so byat this calculation, it wiat rate it'll take another 100 years to complete the series.

So you can'll be able to enjoy it for a long time!

Please read it.

This conveys your meaning, but I would use other phrases (You should read it! or I highly recommend it) to feel more natural.