macOS 10.15 no longer supports 32-bit binaries. See: 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support
Apple’s transition to 64-bit technology is now complete. Starting with macOS Catalina, 32-bit apps are no longer compatible with macOS. If you have a 32-bit app, please check with the app developer for a 64-bit version.
The first 64-bit preprocessor we distributed on macOS was in Dynare 4.5.0. Older preprocessors were distributed as 32-bit in the official Dynare packages. You can either recompile an older preprocessor, choosing a commit after we transitioned the code to 64-bit (you could find this transition in the git history), or you could use an older version of Dynare on macOS by installing macOS < 10.15 on your computer or on a virtual machine. Oh! You could also perhaps try dropping the preprocessor binary from Dynare 4.5.0 into your 4.3.2 distribution. If the output files didn’t change too much between those two versions perhaps it would work with the Dynare 4.3.2 backend…