why getting 404 error for custom set up permalink ?

Hi ,
I am getting confused why I am getting 404 not found error when I am trying to change the default permalink set up which is :
https://mydomain/blog/?p=123
and try to do custom set up as :
/%postname%.html
but I am getting 404 not found error when I browse to my post.
how can I fix this ?
thanks for any advice.