The Move to SD Card option is disabled if you have
android:installLocation="internalOnly"
or if you have not explicitly installed android:installLocation in the manifest file.
According to official documentation
By default, your application will be installed in the internal storage and cannot be installed on external storage unless you define this attribute as “automatically” or “preferred”.
Add
android:installLocation="auto"
or
android:installLocation="preferExternal"
in tag <manifest>
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