Hello, I am currently running 10.13.6. I need to update so I can continue using Aoole Mail.

Your iMac must be at least a 2012 or newer sub-model to upgrade to macOS 10.15. You can quickly find find your sub-model by doing "About this Mac..." from your Apple menu. Example:

If a 2011 or older sub-model, it is hardware-constrained to running no higher the 10.13 High Sierra.

If it is a problem with the old version of Safari linked to 10.13, try using the FireFox browser.

If your iMac is capable of a newer macOS version, you can get them here:

How to download and install macOS - Apple Support

What year model is your iMac?

see > Identify your iMac model - Apple Support

Upgrading to a later macOS version, depends on the year model.

see > Update macOS on Mac - Apple Support

Is this an issue with a Gmail account?