Unable to install datasmith 5.4.2.0 for Archicad 26

You need to uninstall an old verison first.
Use that file to uninstall your 5.0.1 https://unrealstudio.epicgames.com/UnrealDatasmithArchicadExporter_5_0_1.msi

then run the 5.4.2 install.

I recommend you get the 5.4.3 version, when it is released in a couple of weeks, as it comes with some bug fixes.