In this tutorial you will learn how to fix the error "unable to launch "cinnamon-session-cinnamon" X session --- "cinnamon-session-cinnamon" not found; falling back to default" on Linux Mint.
Follow the steps below to fix this issue
Step 1 - Power on your system
Power on your computer and once you see this error, click the Okay button

Next, click the "Log Out" button

Now you need to press CTRL + ALT + F2, this command will launch the console from where you can manage your system without GUI, for example in this case GUI it's not loading so we need to use the console only.
In my case I could see the login screen, that's because I use a username and password to login in my system, if you use the autologin feature where you don't have to type username and password to login then you probably will not see anything else and it would take you directly to the error window. No matter if you see the error or the black screen only, press CTRL + ALT + F2 and proceed to the next step.

Step 2 - Login to your User
Once you have pressed the shortcut keys above, you will see a black window that will ask you for username and password
Username - Is your username
Password - Password of the user you are trying to login
Now simply enter username and press enter

Enter your password and press enter

And you should now be logged into your system, the console should look like the image below, now you should be good to go and proceed to the next step by installing cinnamon.

Step 3 - Install cinnamon
Once you have logged in on your system and have access to console, now you need to install Cinnamon
To install cinnamon simply enter the command below
sudo apt install cinnamon

Once cinnamon desktop environment has been installed, you can reboot your computer and the error should be gone.
Now you can reboot your computer and the error should be gone
reboot
Conclusion
By now you should know how to fix the error "unable to launch "cinnamon-session-cinnamon" X session" on Linux mint.