In Linux (Portuguese www.caixamagica.pt version, although I expect a common solution...) how can I make a key combination to generate the at sign?

Your keyboard layout probably isn’t configured correctly, try going to the keyboard settings and selecting a layout that fits your keyboard.

Also, you’re not just using “Linux”, but a certain Linux distribution, e.g. Ubuntu. It would be handy to know the name of this distribution.

thanks @Vincentt I think I tried the keyb mapping thing already

As for the distro, i mentioned it in the title of this question

www.caixamagica.pt

many thanks!