FileMaker Pro 14 DVD rejects without installing on my MacBook Pro running 10.15.7 . why is this?

I don't know. This Claris support article indicates that there were some Catalina compatibility issues with FileMaker Pro Advanced 18.x, which is several versions newer than the one you're trying to install.

https://support.claris.com/s/article/FileMaker-and-macOS-Catalina-Compatibility?language=en_US

One possibility that comes to mind is that the Filemaker Pro 14 DVD installer might contain 32-bit code. It sounds like around version 14 / 15, Claris was shipping application bundles containing both 32- and 64-bit code. I do not know how Catalina would react when presented with such a bundle.

https://support.claris.com/s/article/FileMaker-Pro-Advanced-and-Windows-64-bit-operating-systems-1503692926871?language=en_US