There were several models of 2011 iMacs. All of them can go as far as macOS 10.13.6 (High Sierra), but no higher. High Sierra is a very old operating system, and this may severely limit the ability to use commercial software from the likes of Microsoft and Adobe.
There are programs you can get for High Sierra that might prolong the useful life of that iMac. For example, the Extended Service Release version of the Firefox Web browser, or the archived 7.4.7 version of LibreOffice.
In the case of Roblox, they appear to support very old systems – including apparently Macs running High Sierra.
https://en.help.roblox.com/hc/en-us/articles/203312800-Computer-Hardware-Operating-System-Requirements