Try pressing F11, Left Shift + Enter, Left Ctrl + Enther or Left Ctrl + Shift + Enther if all those didn't work then i dont know and that is how i full screen most game i played

Assuming you are on PC using Steam, When opening the game from Steam the startup options allow you to open the game configurator. In there, you can reset various options, including opening the game in safe mode. In the configurator option "Full Screen" you can choose Fullscreen, Windowed, or Borderless Window. Choose Fullscreen OR try resetting using the "Default" button to reset all variables which may be messing up your experience. In either situation you try, press "Save" button afterwards and then close out the configurator. Then start the game again in regular game mode and see if it works. If not, open the configurator again and choose safe mode to play. If that still doesnt fix your issue, then the problem may be more severe. May have to reinstall the game, but first try verifying the files in Steam client. If you need to reinstall, then uninstall, and delete any of the game's configuration files left after uninstalling. Make sure not to delete your save game files however. If this still doesnt work, the issue may be system related, or may be connected to your graphics card software. I use NVIDIA, and both the NVIDIA GeForce Experience software AND NVIDIA Control Panel can set graphical preferences for particular games/apps. If you use other graphics software, you can also check those softwares to see if they are overriding your game settings to windowed mode instead of fullscreen. You might want to check your graphics softwares before trying the reinstall method.

Also, if you previously selected the option to always use the same startup option so you cannot open the configurator, you can re-enable the option to choose using the gear icon on the game page in Steam. Upon opening the gear icon dropdown, select "Properties...". IN the window, in the general tab on the left, the right hand panel will show "Launch Options" and a dropdown box below it. Choose "Ask when starting game" and close the properties window. You can now use the "PLAY" button in the game's Steam page to choose the configurator.