You need to define your function for the thickness variation in Analytical/Discrete Field from Fields, & select it for your shell section instead of the default option (Value: ---) in: Sections ---> Shell thickness --> Element/Node distribution. It's also good to try out your model by changing between Mapped/Expression types in case of selecting an Analytical Field over a Discrete Field.

The video below is explaining how to change shell thickness using mapped field. You can watch it for a better understanding of the required steps.

https://www.youtube.com/watch?v=JokMknz3cE4&ab_channel=BWEngineering

Using MAP field, just like Lamprini Papargyri and Sina Ahmadvashaghbash mentioned.