The package either doesn't declare a versioned build dependency on debhelper or does not declare a versioned build dependency on a new enough version of debhelper to satisfy the declared compatibility level.
The required version of debhelper is not guaranteed to be satisfied in all supported releases of Debian and therefore this may lead to a build failure.
The recommended practice is to always declare an explicit versioned dependency on debhelper equal to or greater than the compatibility level used by the package, even if the versioned dependency isn't strictly necessary. Having a versioned dependency also helps with backports to older releases and correct builds on partially updated systems.
Packages not using an experimental or beta compatibility level may alternatively Build-Depend on the debhelper-compat virtual package, for example:
Build-Depends: debhelper-compat (= 12)
Note if you are using a compat level marked as experimental (such as compat 12 in debhelper 11.4~) please explicitly override this tag.
Refer to the debhelper(7) manual page for details.