I'm not sure what to say until I see what you are clicking on and what you are seeing. I've tried multiple different files and I can print them just fine.
Both of these options, Print (Crtl+P), Print to PDF (Crtl+Shift+P), work and I have successfully printed using them:
View attachment 358135
I only get this error:
View attachment 358136
When I try to Save, Save As, or Export. No other option has given me that error yet.
Are you using Pepakura Designer 6, the latest version which I linked above?
https://tamasoft.co.jp/pepakura_designer/ext/get_pepakura6.php
Maybe if you are using an older version of Pepakura Designer, like 4, you might get an error if you try to print as Pepakura used to have two separate versions, the Free "Viewer" and Designer was the paid version. These days, including Designer 6, there is only the one version which should only be locking the save/save as/export features behind the license key.
So, confirm what version you are using as well.