In order to send text messages to attendees, you must have a phone number field on your page and have the SMS Opt-In enabled.
The phone number field can be in the billing section or even added to the ticket block to capture a phone number for each ticket holder.
Enable the SMS Opt In
From the Pages screen, click the pencil icon on the page you'd like to enable SMS Opt-In on
Edit the existing phone number field on the page, or you can scroll down to the billing section and hover your mouse over the Phone Number field and click the pencil on that field
Make sure the field option titled Enabled is to Yes
Click Advanced Options and toggle the SMS Opt In to Yes
Tip: If you have multiple phone number fields, be sure to enable the opt-in for each of them.
FAQ
How does a text message subscriber opt out?
Your attendee's first message instructs them to reply STOP to opt out. This is the industry standard. Their number will be added to a do-not-send list by the carriers. If an attendee sent STOP by mistake or wants to remove themselves from the do-not-send list, they should send the keyword START to resume. When an attendee opts out, they opt out of all future messaging from that number, not just that event.