It's a matter of control
Ticklishness is a warning system built into the human body by evolution.

Some of us have it to a greater degree of sensitivity than others, and some have an ability, either inherent or learned, to control their reactions to tickling sensations, for greater or lesser periods of time.
When you get a low degree of sensitivity combined with a high level of control in the same person, you get the people who seem
😉 not to be ticklish.
In order for a person to really not be ticklish at all, they would have to be suffering from a disability which essentially turns of the tactile sense, either entirely or very close to entirely.

These conditions are medically known, and are sometimes hereditary and sometimes caused by disease conditions. Such individuals should be treated with the same compassion as any other disabled person.
Personally, I have encountered many women who stated, when asked, that they were not ticklish.

The overwhelming majority of them were conciously lying.

Those who really believed it sometimes felt so certain that they allowed me to put them to the test; every single one of those, without exception, turned out to be wrong.

Some of them had been unwise enough to agree to substantial wagers. Understand that I am now referring only to women who had been my girlfriend or lover

before the test took place. I would offer a wager like, if they turn out not to be ticklish after x number of minutes, I'll take her shopping and buy her a new dress, pair of shoes and handbag. If she loses and is ticklish, the next time we make love I have her permission to hold her down and tickle while making love.
There are few things more fun than watching the face of a woman who honestly believed she was not ticklish find out that she was wrong!
Mastertank1
We who play and dance are thought mad by they who hear no music.