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Since Dashing, you need to first declare a parameter before getting or setting it. If you try to get or set an undeclared parameter you will either get an exception thrown, e.g. ParameterNotDeclaredException, or in certain cases you will get an unsuccessful result communicated in a variety of ways (see specific functions for more details).

May I request you to read the documentation at the following URL:

https://docs.ros.org/en/foxy/Releases/Release-Dashing-Diademata.html#getting-and-setting-undeclared-parameters