Why doesn't my keyboard repeat a letter I press?

The question might not be very explanatory… Sorry for that.

Whenever I press a letter and keep it down for a while it’s not repeated. I have to press the letter ‘a’ 10 times more if I want to write “aaaaaaaaaah” for example.

Any idea how I can turn this option on, on my Mac? I want to keep a letter pressed and let it appear for as long as I hold my finger over the key.

Thanks in advance!