Mac needs to be updated but it doesn't show any updates

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Sierra download

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra/Mojave or find an older computer where you can download it using your Apple ID.

El Capitan

Check to make sure your applications are compatible.        

Application Compatibility

El Capitan 10.11 Compatibility information 

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Microsoft Office365 has a three-year sliding window of Mac operating systems that it will support. RIght now:

• 10.14.5
• 10.13.6
• 10.12.6

When Catalina ships this fall:

• 10.15.n
• 10.14.n
• 10.13.6

n = current or last version.

So, your Yosemite is not supported by Office365, and very shortly that advice to update to 10.12.6 (Sierra), will change to 10.13.6 (High Sierra), because Sierra will not be supported by Office365 either.

From the menu : About This Mac panel, behind the name of your Mac is a parenthetical product introduction date (e.g. Late-2013). What does that entire line read? Based on that, I can tell you what your macOS upgrade path can be.

Can you click the Apple at the upper left, then select "About this Mac" and report the number next to "Version"? You might need to update to a new OS completely.

How are your running Yosemite? iMac Pro’s ship with v10.13 High Sierra, I don’t know how you could be running Sierra or El Capitan.