my mac will not let me update

Please post the Mac model and year from 🍎 > About This Mac…, and with both the model and the model year, we can provide the available upgrade options.

Or if you want to research this yourself, start here: How to get old versions of macOS - Apple Support

Have a backup prior to the upgrade, if your Mac supports Mojave or later.

If you’re on the bottom-end 8 GB / hard disk drive iMac 21.5” model, performance tends to be lacking on more recent macOS versions. An iMac with SSD does much better, either internal or booting from external SSD.

Stevemusa447 wrote:

i have a imac 21.5 inch 2011 20GB memory 1333 MHZDDR3 2.7 GHZ intel core i5

Both iMac 21.5” mid 2011 and iMac 21.5” late 2011 models top out at macOS 10.13 High Sierra.

macOS 10.14 Mojave and later are not supported on that Mac.

There are no newer versions for that iMac.

Microsoft supports their apps on the three most recent releases on macOS, same they do over on Windows.

You will need to use a different mail client, or a different Mac, or—if it’ll even install and boot here—install Windows 10.

Maintaining and supporting and updating old software is not free for the vendor.