I asked

My issues are the following:

1. Trigger/execution error: Exception: Ranges must have at least one range. at email(Code:103:15)
2. The email notification contains information from the last row of the sheet, moving up, rather than the row with the new user which is the row(s) where there's no "Y" under the "Sent" column, Column I. The "Y" is added to this column once the script executes and sends out an email. (Ex., if I receive 3 notification emails for 3 new users, the data in the email has data from the last row, the next email has data from the second to last row, and the third email has data from the third to last row in the sheet).

You may want to try using it as a Time driven trigger instead. This is how the latest test emails were received:

This is how the emails were received in my mailbox:

This is how the execution is displayed in the logs:

As you may notice there are no errors shown in the logs so I would recommend using Time-driven triggers instead.