When someone says “stop tickling me” the person doing the tickling should stop tickling them.
Not stopping physical touch when someone says “stop” is a boundary violation.
It can be abusive if the person tickling doesn’t stop when asked. Some people aren’t very ticklish themselves and don’t understand that it can be agonizing for a ticklish person when they are unable to catch their breath from laughing so hard. It is important for kids, or for anyone, to know that their bodily autonomy will be respected and that they have the right to tell anyone, parent or not, not to touch them in ways that are uncomfortable (even if something as seemingly innocent as tickling).
Children laugh when they are being tickled because tickling activates the brain’s fight-or-flight response and pain receptors, and laughing in response to tickling is an instinctual submission response to the perceived threat.
Your child, therefore, cannot help but laugh when tickled, as her brain perceives an overpowering threat and instinctively submits through laughter.
If the child is screaming “stop it” then that means “stop it.”
And you know how they feel, because you felt that way yourself when you were a kid.