when I go to upload sketch I get bad cpu in executable? how do I fix this?
Uninstall the version of IDE you are using. Then install 1.8.10, and in tools -> board manager, ensure that the latest version of the Official AVR Board Package is installed (assuming it's an AVR you're uploading to). Latest versions of official megaavr board package, and my two cores, also definitely work. Other third party board packages may not have updated to support catalina yet.
(cause of issue is that until now, the toolchain distributed with the IDE and board packages was 32-bit; catalina drops support for 32-bit applications)
It’s a Leonardo board on Catalina
Then update to 1.8.10 of the IDE, and use board manager to upgrade to latest version of Official Arduino AVR board package.