In OS Catalina make a account Standard

Solution 1:

You don't need to use Recovery/Single User mode for this.

If you don't have a new admin account yet, create one in System Preferences->Users & Groups
Log out of the current account (the one you want to switch from admin to standard)
Log in with another admin account
Open System Preferences->Users & Groups, select the old account and untick "User can administer the computer"