Boot Mac in recovery. Reinstall Mac OS. Mac OS Catalina. 2018 13” with touchbar. Shows Macintosh HD. Click it. Click install. Not connected to power source. Click continue. Says the target volume is part of incomplete system and can’t be installed

You do need to be booted from another drive in order to erase the physical drive. Either from a bootable backup, a bootable macOS USB installer, or from Recovery Mode or Internet Recovery Mode. I personally always erase the whole physical drive in order to refresh the partition tables as well.

If you want to reinstall the OS that originally shipped with the computer, then you either need to create a bootable macOS USB installer if it is listed in that Apple article or you need to boot into Internet Recovery Mode using Command + Option + Shift + R which should boot the installer for the original macOS which shipped with the computer or the oldest version of macOS still available to download.

You can create a bootable macOS USB installer to keep on hand:

https://support.apple.com/en-us/HT201372

Or you could install macOS to an external drive which you keep on hand for emergencies so you can boot the spare drive externally.

You can also make a bootable clone using Carbon Copy Cloner.

Daniel Moilanen wrote:

You only need to be booted from another drive if you want to erase the entire drive. I don’t mean that. I don’t want to mess with the recovery partition so I’m talking about erasing the rest. Not the entire drive.

What I meant was you cannot run the macOS installer from within macOS to erase the drive since you will be destroying the installer located on the drive in the process. I personally feel if you are going to erase the boot volume you should refresh the whole partition table just to be safe. I want to eliminate all possible sources of issues at one time when reinstalling. But that is my opinion and personal preference.

Booting into recovery to install the factory default is doesn’t work. It only has worked for a short time after the Mac came out. Otherwise it just installs the newest os which I do not want. I want to go back to Sierra which is what this came with I think.

There are three different key combinations for entering Internet Recovery Mode:

• Reinstall the current or last installed version of macOS (doesn't always work as intended and will sometimes install the oldest supported OS instead):

Command + R

• Install the original version of macOS which shipped with the computer (or the oldest version still available):

Command + Option + Shift + R

• Install the latest supported version of macOS (at the moment for your laptop it will be Catalina):

Command + Option + R

In my personal experience Internet Recovery Mode is broken for many systems and it will default to the original or oldest version of macOS still available for a particular computer. I've never had Internet Recovery Mode default to the most recent version of macOS, but I guess it is possible.

The option I highlighted here in bold (which I also presented in the previous post) should provide you the installer for the version of macOS which originally shipped with your laptop. In my previous post I also provided you a link to the Apple article for creating a bootable macOS USB installer for macOS versions 10.11 to 10.15.

I even want to go older and put on here last version with the 3D dock.

You cannot install a version of macOS older than what originally shipped with your computer.

If you selected "Show All Devices" the top item should be the physical drive which should show up something like "Apple SSD....". I have not done this on a 2018+ Mac. I don't think any security settings will prevent you from seeing or selecting the physical drive if you are booted using Recovery Mode. Double-check that the "Show All Devices" setting actually took and wasn't accidentally cancelled due to trackpad touch issues.