Blank Sudoers File

I know that Todd said that it wasn't smart to do this, but I went to the opensource sudoers file from apple and copied that into my sudoers file, now sudo is working just fine!

1. Boot to recovery mode
2. Open Terminal (menubar > Utilities > Terminal)

3. cd to your system data volume:

   cd /Volumes/Macintosh\ HD\ -\ Data/private/etc/
   

   (In system 10.14 and lower this is /Volumes/Macintosh\ HD/private/etc/)

4. Download a default macOS sudoers file (e.g. from an online repository)

   curl -LJO https://gist.githubusercontent.com/keith/9061156/raw/1f056fa364640900eb42fcc9023d60a759c668c8/sudoers
   

5. Check permissions:

   ls -l /Volumes/Macintosh\ HD/private/etc/sudoers
   -r--r-----  1 root  wheel  1283 May 13  2020 /private/etc/sudoers
   

6. Boot to your main system volume

7. Edit the sudoers file with sudo visudo and replace it with the content of the default Catalina sudoers file:

   #
   # Sample /etc/sudoers file.
   #
   # This file MUST be edited with the 'visudo' command as root.
   #
   # See the sudoers man page for the details on how to write a sudoers file.
   
   ##
   # Override built-in defaults
   ##
   Defaults    env_reset
   Defaults    env_keep += "BLOCKSIZE"
   Defaults    env_keep += "COLORFGBG COLORTERM"
   Defaults    env_keep += "__CF_USER_TEXT_ENCODING"
   Defaults    env_keep += "CHARSET LANG LANGUAGE LC_ALL LC_COLLATE LC_CTYPE"
   Defaults    env_keep += "LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME"
   Defaults    env_keep += "LINES COLUMNS"
   Defaults    env_keep += "LSCOLORS"
   Defaults    env_keep += "SSH_AUTH_SOCK"
   Defaults    env_keep += "TZ"
   Defaults    env_keep += "DISPLAY XAUTHORIZATION XAUTHORITY"
   Defaults    env_keep += "EDITOR VISUAL"
   Defaults    env_keep += "HOME MAIL"
   
   Defaults    lecture_file = "/etc/sudo_lecture"
   
   ##
   # User alias specification
   ##
   # User_Alias    FULLTIMERS = millert, mikef, dowdy
   
   ##
   # Runas alias specification
   ##
   # Runas_Alias   OP = root, operator
   
   ##
   # Host alias specification
   ##
   # Host_Alias    CUNETS = 128.138.0.0/255.255.0.0
   # Host_Alias    CSNETS = 128.138.243.0, 128.138.204.0/24, 128.138.242.0
   # Host_Alias    SERVERS = master, mail, www, ns
   # Host_Alias    CDROM = orion, perseus, hercules
   
   ##
   # Cmnd alias specification
   ##
   # Cmnd_Alias    PAGERS = /usr/bin/more, /usr/bin/pg, /usr/bin/less
   
   ##
   # User specification
   ##
   
   # root and users in group wheel can run anything on any machine as any user
   root        ALL = (ALL) ALL
   %admin      ALL = (ALL) ALL
   
   ## Read drop-in files from /private/etc/sudoers.d
   ## (the '#' here does not indicate a comment)
   #includedir /private/etc/sudoers.d
   

Hint:

You can access this question & answer in Recovery Mode with Safari and copy the curl command.