This is most likely because the name and/or email address you are trying to change it to already exists on another customer profile.
You can use the Search bar at the top of the page to check if that name/email is attached to another customer:
If it does exist on another customer profile, there are a few options:
a) Change the email address on the other customer profile, in order to free up the correct email address to be used on this customer profile. NOTE - these customers will appear with these updated email addresses in reporting
b) If you are changing a customer email address so that they can receive ticket emails ,and you don't want to do the above, then you can always send ticket PDFs to the customer directly. To do this, click Print As or Download PDF on their order page and send them the ticket PDF file by email: