The error 'Unity Engine Input System Missing' typically occurs when the new input system package by Unity, known as
InputSystem, is not correctly installed or configured in your project. There could be a few reasons behind this:
To resolve the 'Unity Engine Input System Missing' error, follow these steps:
Install the Input System Package: Open your Unity project and go to Window -> Package Manager. In the Package Manager, select 'Unity Registry' from the dropdown list on the left, scroll down to find the 'Input System' package, click on it, and then click 'Install'.
Activate the Input System: After installing the package, Unity will need to switch from the old input system to the new one. Typically, Unity should prompt you to do this automatically but if it doesn't, you can manually switch the systems: go to Edit -> Project Settings -> Player -> Other Settings and set 'Active Input Handling' to either 'Both' (if you want to use the new and old systems together) or 'Input System Package (New)'.
Restart Unity: After making changes to the input settings, restart Unity for the changes to take effect.
Should the problem persist even after following these steps, consider verifying your Unity installation or reinstalling Unity to ensure there are no issues with the installed files.