Hi Bret,

TeamCity version you are using is outdated and no longer supported. Please upgrade to the latest version available (2020.1.5). It should work fine even with Big Sur.

The error you are facing likely happens because your TeamCity runs on a 32-bit JVM. Before TeamCity 2019.2, we bundled 32-bit Java with the application, but your version can work with 64-bit Java 8. Check this section of your version's documentation for more details. Although for your version it is recommended to use the 32-bit version, you can run TeamCity with 64-bit Java, and that should solve your problem.

Note, that if you are currently using the bundled Java (the one under jre folder in TeamCity home), it will not be upgraded along with TeamCity and you will need to manually install the recommended version. For the latest (2020.1.5) version, we recommend using 64-bit Java 11. See this page for more details.

Here is the real answer to this question.

Agent launcher is missing wrapper libraries for 64-bit macOS (do not use wrapper on MacOSx)
https://youtrack.jetbrains.com/issue/TW-59368

I have TeamCity server running on Windows server and would like to run an agent on Mac.
As I stated above the BuildAgent.zip from the server does not have a 64-bit version of TeamCityAgentService-macosx-universal-32
I had checked the BuildAgent.zip from version 2020.1.4 and it still did not have 64-bit.
You stated that 2020.1.5 should support it, so I updated and licensed it (since I have 150 build configurations and 5 agents), but it still did not have 64-bit Mac agent.
I finally found this reference to the fact that they are now skipping the Mac agent wrapper, so the way I was trying to configure it was wrong/would never work.
TeamCity should definitely highlight this fact in the setup instructions for setting up an agent on MacOS.