There is no concrete answer, but by looking at the characters in the Witcher, not everyone can learn these signs since you require specific magical ideation to use them. Most people have small magical prowess, but some are born without it in the Witcher Universe. You also need hand gestures to use the signs since, in The Witcher 1, we found that since Leo broke his fingers, he couldn't use the signs. Ciri, who was trained as a Witcher but didn't undergo mutations, couldn't use the signs. It is said that certain potions can allow magic pathways to open up within people allowing them to use the signs for a limited amount of time.