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Unity's "New" Input System
This article will walk through the steps to enable and Disable the new Input System and the common troubleshooting issues
Install the new input system from the Package Manager. You should do this before you change your project settings.
Doing this (installing the Input System) should cause Unity to toggle the New Input System on for you so you can usually skip Step 2.
Enable the new input system in your project settings.
Usually Unity will do this for you when you install the Input System as done in Step 1 so this should already be done for you.
Remove the Input System from the Package Manager ... we like to do this first even though it may throw a compiler errors for assets that use both new and old; we find it faster/nicer to remove it before toggling the setting as the setting will force a restart.
As noted this will cause two exceptions or more to be thrown ... this is expected and will be resolved in the next step.
This happens because your settings still say to use the New Input System ... but you dont have the New Input System installed.
Toggle your Player Settings to use the "Old" Input System.
This will restart Unity