No "Encrypt Phone" option on Android 7.0 Samsung Galaxy S7
Thanks to Samsung non standard implementation, OP couldn't factory-reset and encrypt before selling. They couldn't :


Check if the device was encrypted
Locate the option for encryption


Steps advised :


Install minimal adb on your laptop
(Is there a minimal installation of ADB?) and check device encryption status (Is there a way for a user to tell if their Android device is encrypted or not?). Alternatively, install Termux on your device and type getprop ro.crypto.state
Encrypt device from Settings &rarr; Lock Screen and security &rarr; secure start up &rarr; require password (Source)
factory-reset after signing out from Google account, else you will have problems with factory reset protection and buyer will not be able to use your device
Confirm device is encrypted. Decrypt device (to remove lock screen PIN ) and sell :)