Has anyone upgraded to Catalina on a mid 2011 imac? I have upgraded memory (16gb) and have plenty of sstorage. . .cannot download due to "incompatibiity" but I cannot get Apple tech support to tell me what the incompatibility consists of.

One needs a 2012 or later iMac to upgrade to Catalina. A mid-2011 iMac does not meet the minimum hardware requirements, which in your case would be the lack of a Metal capable GPU that Catalina requires.

It's Hardware limitation, the Microcode on earlier Intel CPUs cannot be patched for current security problems, then GPU features are not present in older GPU cards that are needed for speed in today's bloated world.

The reason you cannot load Mojave or later version of Mac OS is that they require Metal. Metal is a component integrated into the GPU. Metal did not become available in iMacs until the 2012 iMacs were released. Before you ask, no you cannot upgrade your GPU to one that has Metal and no there are no work arounds. The furthest a 2011 will go is 10.13.6, if you want or need Catalina it is time to buy a new Mac. Something you should consider as your computer is quickly approaching 10 years old, this means software will become outdated (you are already seeing this with Mac OS) and if it has a HD installed, that is way past it's prime and working on borrowed time as are other irreplaceable components.

I also have a 2011 27" that is being replaced very soon. I placed an order for a 2020 27" (actually 2 of them) and am waiting for it to arrive later this week.