Its a short guide to help those who have the same issue i had. The problem is that the Fullscreen mode button is blocked, like this:

(i took this image from another post)

So, i found that the problem was my screen resolution, i have a 1366x768 screen, but the game only works at 1260x768. So, all that i did was to change my resolution to 1280x768 (you manage it in the nvidia control panel, idk about other video cards), then open the game, turn it into Fullscreen mode, close it, change your resolution back to the one that you had, and its done. Then the game will be in fullscreen mode forever.

Im from Venezuela, so, really sorry if you can't understand, my English is kinda basic.

This is how i solved my fullscreen mode issue, if you can't fix it through this method, im sorry

Saludos. (greetings in spanish ^w^)