There are many ways to align text in the center of the email editor but the best and the easiest way is to use a text box and by making changes to its spacing properties.
Once the text box is added, the font is set and alignment is set to the middle, scroll down the screen to view the "Spacing" section. Then edit the "spacing properties". Below is the given screenshot which will guide you where you will get the spacing option in the email editor. Change the left and right spacing to 190 or until you get the required text alignment.
Output on email: