Did you install Ant? Is this the Jenkin WAY? If not, Jenkins cannot use the default version of Ant.
However, if you go to the Jenkins configuration page, you can install different versions of Ant. For example, you can install the latest version of Ant and Ant 1.5 for one project that cannot use any version of Ant outside 1.5:

When you define your project, you can choose the version of Ant you want to use. When you choose that you want to call Ant as a build step, you are presented with a drop-down menu which version of Ant you want to use. The first is that Defaultwhich is the version of Ant found in your PATH. However, if you entered the Jenkins configuration and downloaded other versions of Ant, you can use one of them.

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