Weird error when installing libpng12-0_1.2.54 on Disco

[Ingvar]

I'm trying to install libpng12-0_1.2.54-1ubuntu1.1_amd64.deb on Ubuntu Mate 19.04. This is to satisfy the really old printer SW from Canon :(. It was possible to install on this HW with 18.10 in early April, and it was OK after upgrade to 19.04. Now that I'm doing a fresh install of 19.04 I get this error:
"dpkg: fel vid hantering av arkivet libpng12-0_1.2.54-1ubuntu1.1_amd64.deb (--install):
 kunde inte installera ny version av "/lib/x86_64-linux-gnu/libpng12.so.0": Filen eller katalogen finns inte"
Or in English: "could not install new version of "/lib/x86_64-linux-gnu/libpng12.so.0": the file or directory is missing".

But the directory is there:
$ ls -l /lib/ | grep x86
drwxr-xr-x 106 root root 81920 jul 6 12:55 x86_64-linux-gnu
 and I also downloaded the .deb file again, in case some odd file corruption had occurred.