what is the reason for war between Russia and Checnya?

Answer 1

1) Chechnyans want their own country.
2) Russia doesn't want them to.

Just like Kurds & Basques. Chechnyans, Kurds & Basques are all legitimate nations who want their own country. The world doesn't care.
"Palestinians" are a made up people who say they want their own country & the world condemns Israel. Go figure.

Answer 2

The Chechnyans are accusing the Russians of stealing their cow, which the Russians deny. Also, to complicate things further, the Russians haven't paid their tab at the Chechnyan vodka distellery in a few years.