cannot upgrade imac yosemite OS 10.10.5

Because that’s the last update available for Yosemite.

If you want to do what Apple calls an upgrade, you will be able to upgrade to El Capitan, maybe newer.

For info ➡️ How to get old versions of macOS - Apple Support

Have a complete and current backup before the upgrade.

Use only Safari for the installer download.

If your Mac supports it and you decide to upgrade to Catalina or Big Sur, you will need to ensure all your apps are 64-bit and not 3w-bit. You can use the Go64 tool to check for those older 32-bit apps. For example, Microsoft Office 2011 and earlier will not work on Catalina and later, as these older versions are 32-bit apps. You’ll need Office 2016, Office 2019, the upcoming Office 2021, or the Office 365 subscription, or an alternative such as LibreOffice, or Apple iWork apps.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina & Big Sur support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

SlowLearner123 wrote:

My reservations are economical in regards various Office alternatives requiring my own exploration.

LibreOffice, and the Apple iWork Pages, Numbers, and Keynote are all free.

If you're using Office 2011 or earlier, that'll mostly work up through Mojave, but will not work on Catalina, Big Sur, and later.