How to boot into Single user mode on 2018 MacBoo Pro? Cmd-C does not work.

... and this boot drive is physically installed internally in your notebook? Not sure I understand the difference between a "Base System" and "main boot" drive ... unless "Base System" is the recovery partition on this drive. Is this drive formatted in APFS?

Command-V gets you into 'verbose mode' which looks like the old single-user (command-S) used on older Macs and lets you enter UNIX commands. Perhaps that would work for you?

Start up your Mac in single-user mode or verbose mode - Apple Support

Please read that page before trying it.

Paddle365 Said:

”Cmd-C does not work. Running Terminl in Rewcovery mode doesnot work. I want to run fsck on my main boot drive. How can I do that?”

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Reset the SMC and NVRAM:

Sometimes operations are performed and then the keyboard’s settings and operations become manipulated, technically. So... Attempt resetting the SMC and NVRAM.

Looks like you have already performed the proper procedure to enter Single-user mode on a T2 chip-equipped Mac. Since this is not working for you, and I assume that you gave this a few tries, I believe your only option is to make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to them take a look to see if they can help.

Curious, at which point does it fail to work:

• Your notebook will not enter Recovery Mode after using the <cmd><R> key combination at boot up?
• It does enter Recovery Mode, but you are not able to either select or run the Disk Utility?
• You can access the utility, but can't mount the drive?
• You cannot access the Terminal app?
• You can access the app, but cannot enter any commands at the prompt?
• Something else?