good morning, I can't software update my Mac Sierra 10.13.6 and it doesn't let me install Office 365 since it needs a newer version. But when I click on the top left corner apple icon I follow the instructions on how to update it but the update icon

What model year is your MacBook Pro (from Apple menu > About This Mac)?

A mid 2010 through late 2011 MacBook Pro can be upgraded to High Sierra 10.13.6 but no later version. See: https://support.apple.com/en-us/HT211683

A mid 2012 through early 2013 MacBook Pro can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A late 2013 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475

An early 2015 or later can be upgraded to the current version of Monterey. (later today)

In addition to the useful information, above, I offer the following information. The steps you followed are to update a current operating system to a recently released newer version, such as updating from v. 10.3 to v.10.4.What you are trying to fo is called an upgrade. That is when a newer OS is installed such as upgrading from Catalina to Big Sur. They are completely different things. Upgrades can normally be found in the Apple App store.