A core difference between the administrator account of the user and this built-in administrator account is that the former receives UAC prompts while the latter does not. The user account is an unelevated administrator account while the built-in administrator account an elevated one.

To enable the Windows 10 administrator account do the following:

Tap on the Windows-key. This should open the start menu or bring you to the Start Screen interface depending on how Windows 10 is configured on the system.
Type cmd and wait for the results to be displayed.
Right-click no the Command Prompt result and select "run as administrator" from the context menu.
Run the command net user to display a list of all user accounts on the system.
To activate the inactive administrator account, run the command net user administrator /active:yes
If you want to enable the guest account as well run the command net user guest /active:yes

The administrator account is active now which means that you can sign in to the system using it. It is not password protected by default which means that anyone with access to the system can use it to sign in to it.

It is highly recommended to protect it with a password. This can also be done on the command line:

Type net user administrator * and hit enter.
You get a password prompt. Type the desired password and again when you are asked to type it a second time for confirmation.

To disable accounts at any time, use the following command:

On an elevated command prompt: net user administrator /active:no

Other commands of interested are:

Net user administrator - Displays information about the user account on the command line.
Net user username /delete - Deletes username on the system. This does not work with built-in accounts.