Regnum expects an outdated library. You can solve this by pretending to the game it exists by creating a link from the new one.
For example, on Ubuntu 20.04:
Code:
ln -s /lib/x86_64-linux-gnu/libncursesw.so.6 /lib/x86_64-linux-gnu/libncursesw.so.5
The path /lib/x86_64-linux-gnu/ might be different depending on your distribution.