Hey there! Some apps can't be moved to the SD card. What apps have you tried to move? Or, is it grayed out for all the apps?
Unfortunately, on Android you cannot set the SD card as your default location to install apps, you have to do this manually every time you install an app. You will only be able to move apps which are specifically designed to allow being moved to an SD card. To free up space on your phone, you can try uninstalling apps that you no longer use.
We're sorry but applications are installed on the internal memory by default. You will not be able to set the microSD card as your default location. @kdcain
Go into settings>about phone>software info
tap "Build number" 7 times
go back to settings>developer options
under the subsection APPS, there should be an option to "force allow apps on external" that should un-gray a few buttons.