which one is correct

"it is threrefore,"

or

"it is, therefore,"

You haven't given a complete sentence, and whether or not you use commas depends on what is being said.

You use commas to surround words such as 'therefore' and 'however' when they are used as interrupters.

In these sentences, 'therefore' and 'however' interrupt the main idea of the sentence.

I would, therefore, like a response.
I would be happy, however, to volunteer for the Red Cross.

compare:
Chocolate is made from a bean. It is therefore a vegetable.

"Cholera can kill in a few hours. It is therefore vital that specialist treatment centres are accessible."