how can I get spell cheque?

--  It is "Spell Check".  The word "Cheque" is the British spelling for the monetary voucher a person writes to pay for something, and Americans inaccurately spell as "check".

--  Americans also use the word "check" when they mean "bill" ---  the cost of a meal etc.

--  Vivian has given an excellent answer.

1.) First, make sure your settings are correct by going to Tools > Spelling and Grammar > Options button and make sure the box Check spelling as you type is checked.

2.) If this fails, go to Tools > Language > For all Text and select your language.

I never learned how to install the Google’s grandma checker...

Don, spell check is an option on your internet browser. On Firefox, the one I'm using at the moment, I'd click on the "hamburger" of three horizontal lines, which opens a drop-down menu, them click on options, and "check spelling as you type" is among the many suggestions.