I AM UNABLE TO UPDATE MY MACBOOK PRO

If you really mean upGrade - that is different than update. Update is within the same version of macOS like El Capitan 10.11.5 to 10.11.6. The Upgrade is moving from El Capitan to High Sierra 10.13.6

Since your posting is in High Sierra and your profile is MacBook Pro 15″, OS X 10.11 - we are guessing High Sierra is desired.

The Age of this computer will Govern what newer version of macOS it would Qualify for. About this Mac will provided the needed information for you. Then head over to How to get old versions of macOS where direct links to Verify your computer Age versus what version of macOS it will qualify for.

Once you have determined what version is compatible - Use Safari to start the Download - do not use Others as then make not work. Plus, make a Time Machine Backup Before attempting the upGrade.