An axis inside a sketch is just like a construction line - it is only usable within the sketch and is not available outside the sketch.

Unless you make it an "Output feature"

Edit the sketch, and right-click on the axis line and then select the option Output Feature. Exit the sketch. Now you can publish the axis line, by selecting it in the tree. It's hidden in the tree under the sketch branch in a sub-branch called "Outputs." Don't forget to rename the published line.