Unless you are running i5/OS V7.1 or higher, or you have a “home-grown” or 3rd-party DLTOBJ command, there will be no DLTOBJ command anywhere on your system.
If you are running v7.1 and the command can’t be found in *LIBL, you need to call IBM Support to determine either the reason the command cannot be found or how to recover it. It might have been renamed or damaged, or some other unknown condition might have arisen.
If you have a “home-grown” or 3rd-party DLTOBJ command, then either your library list needs to be changed to include the library that contains the command or you need to qualify the command with the appropriate library.
It’s also possible that you (or the job user) aren’t authorized to use the command. That possibility might result in the message depending on exactly what program is sending the message. That seems unlikely since it is within a CL program, but it needs to be mentioned.