Source contains one or more files not licensed in compliance with the [[ https://www.debian.org/social_contract#guidelines |DFSG ]].
Either the already existing license needs to be clarified in the file debian/copyright, the copyright holder(s) need to be contacted to grant a license (quoting an email from them is sufficient), or the source needs to be repackaged with the problematic files removed (and build rules then adapted if needed).
Most likely source is not in compliance with [[ https://www.gnu.org/philosophy/free-sw.html | FSF ]] guidelines either (that is not tracked specifically, however - but for stuff DFSG-free but not FSF-free see T234)
Ideally, all bugs of this class are [[ https://bugs.debian.org/cgi-bin/pkgreport.cgi?users=debbugs@pureos.net;tag=nonfree-license | reported upstream to Debian, with appropriate usertag ]].