I have a similar problem (more than 4.000 repeated error messages)
depmod: ERROR: ../libkmod/libkmod.c:515 lookup_builtin_file() could not open builtin file '/lib/modules/5.6.0-050600rc4-generic/modules.builtin.bin'
installing kernel 5.6.0-050600rc4-generic downloaded from https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.6-rc4/
I have a similar problem (more than 4.000 repeated error messages) libkmod. c:515 lookup_ builtin_ file() could not open builtin file '/lib/modules/ 5.6.0-050600rc4 -generic/ modules. builtin. bin' -generic downloaded from https:/ /kernel. ubuntu. com/~kernel- ppa/mainline/ v5.6-rc4/
depmod: ERROR: ../libkmod/
installing kernel 5.6.0-050600rc4