Source contains one or more files not licensed in compliance with [[ https://www.gnu.org/philosophy/free-sw.html | FSF ]] guidelines.
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).
NB! For source not in compliance with the [[ https://www.debian.org/social_contract#guidelines |DFSG ]] (and possibly FSF guidelines too) see T233.