In a recent update () in Ubuntu 18.04 normal system operation (Dynamic Kernel Module Support), used to build third-party kernel modules after updating the Linux kernel.
A symptom of the problem is the output of the message
"/usr/sbin/dkms: line### find_module: command not found"
when manually installing modules, or suspiciously different sizes of initrd.*.dkms and the newly created initrd (this can be checked by unattended-upgrade users). It is important that the problem is not a reason for the batch script to stop and report an error without corrupting other initrds.
For testing already corrected version of the dkms package. To avoid problems when using DKMS, before the release of the fix in stable versions of packages, it is recommended to temporarily disable the automatic package update script.
Source: opennet.ru