Why was shays rebellion considered to be the united states first civil war

It's was the first rebellion again the new US government. Soldier who fought in the revolution had not gotten paid yet so they stood up again the continental congress. The founders didn't not like this so they crushed it.

Facing the loss of his own farm, Shays organized a rebel armyof over 1,000 followers which stormed a Springfield debtors court and arsenal, determined to attack and overthrow the government in Boston. Though the rebellion was ultimately unsuccessful,it had profound results that changed America forever. In the wake of the Shays’ rebellion, the states moved quickly to hold a Constitutional Convention that eventually led to the drafting of the Constitution and later, the Bill of Rights. In many ways, it was Shays' Rebellion that persuaded General George Washington, who had retired after the war, to return to politics and accept the crucial role of the nation’s first president.
Hope this answer helps! feel free to ask any additional questions :)

Also if you are good at Geometry, would you mind going and seeing if you could answer a few of them?