Thanks Smithy, that WAS the problem. Something must have reset because I haven't used it in a few months, but I've never had this issue before.
It can't remember if El Capitan already has the security feature where you can allow an application full disk access, but check the systems settings - security. In Catalina (alsi with Mojave) there is a settings to allow full disk access to applications. Then you should be able to save it where ever you want.
I guess it is the sandbox feature of OSX and you try to save the file in a directory where Cura has no permissions to save something. Have you tried to save the file to the desktop for example?
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