Can you first confirm for me if you are using Flipbook Printer (older) or FlipSuite (newer)?
Can you screenshot or tell me what kind of error you are getting with the card stock template and what triggers it? I do remember at some point an older error having to do with using , instead of . for fractional numbers..
Regarding the license key: I sold the commercial rights to FlipSuite many years ago and the agreement we made that let me keep it freely available was that the license keys were no longer needed and the program could be used forever, but the words "for non-commercial use" would be printed in the margin on the books. So the short answer is that you don't need to put in a license key to the program.