I installed the mod, but the GUI doesn't come up. How to turn on GUI?

Per player shortcut to toggle the GUI on and off with the image of a bullseye.

https://mods.factorio.com/mod/rocket_target#:~:text=Per%20player%20shortcut%20to%20toggle%20the%20GUI%20on%20and%20off%20with%20the%20image%20of%20a%20bullseye.

I am sorry. I looked for it, but I don't know where it is. Is it somewhere on the game screen? Please tell me the location. Thank you.

Shortcuts are the things to the right of the quickbar at the bottom of the screen.
Wiki page on "shortcut bar": https://wiki.factorio.com/Shortcut_bar

Either in the shortcut bar or under its 3 dots (configuration menu) there will be a bullseye symbol that toggles the counting GUI on/off.