If you are using SMF 2.0.x, and a package is shown as installed when it is not, or is shown as uninstalled when it is installed, you can use this tool to fix it: Fix Packages.

Copy this file to your SMF forum main directory, where SSI.php and Settings.php are located. Next, direct your browser to the file fix_packages.php in the same directory as your forum. Select the package and change it to installed or uninstalled.

Do not click the Remove all hooks button unless you know for sure all your mods are uninstalled.

Afterwards, with the installation status set properly, you can attempt to install/uninstall the mod again.