You need to set the path correctly.and also if you are facing problem like it is now working when the terminal is reopened then you should try this method and simply restart your computer after doing.
open your terminal and type
nano ~/.profile
and add this line end of the file and change Add the following line and change [PATH_TO_FLUTTER_GIT_DIRECTORY] to be the path where you cloned Flutter’s repository.
export PATH=[PATH_TO_FLUTTER_GIT_DIRECTORY]/flutter/bin:$PATH
For example i am showing you what i have done in my case.
Now simply ctrl + O and hit enter
then type in your terminal
source ~/.profile
and to check your path just run
echo $PATH
now just simply type flutter
in your terminal. hope it will work.