Can't update my iMac to Ventura. IMAC says I'm upto date with Monterey 12.7.6.

In order for your iMac to be upgraded to macOS Ventura, it must be one of these models:

iMac (24-inch, M1, 2021)

iMac (Retina 5K, 27-inch, 2020)

iMac (Retina 5K, 27-inch, 2019)

iMac (Retina 4K, 21.5-inch, 2019)

iMac (Retina 5K, 27-inch, 2017)

iMac (Retina 4K, 21.5-inch, 2017)

iMac (21.5-inch, 2017)

Ref: macOS Ventura is compatible with these computers - Apple Support

If you are not sure what model you have, select "About this Mac" from the Apple menu at the top left.

Note that it tells not just "iMac" but also the year (e.g. "iMac (late 2015)").

Depending on the year, it may well be that you have the last OS that is supported - see the list that Tesserax posted already.

Exactly what model Mac do you have?