evaluation – Mathematica 12.1 is treating all my cells as raw texts by default

I was once using Mathematica 11 it all works fine – when I create new Notebook (.nb) input like 1+1 and press shift+enter, it shows output(1):= 2 directly

But now for 12.1 when I create new notebook and do the same thing, it is treating my input 1+1 as plain text by default, when i press shift+enter there is just a linebreak.. no evaluation at all.

I find when I go to the menu -> cell -> cell properties and tick Evaluatable then the input can work well as Mathematica 11. BUT when i input Plot(x,{x,1,2}), it gives output -Graphics-!! Without showing what the graphic is! I then have to manually convert the output cell to StandardForm to show the graph! I have to do such things everytime, why is 12.1 getting me so much ados ..and how can i remove such tedious things making it funcation just like what mathematica 11 does?

Pardon me I’m a complete nooooooob and really searched goooogle tons of times not getting the answer….