Make sure you entered a valid US mobile number. International numbers aren't supported.
Check that the number isn't a VoIP or landline (texts won't deliver to those).
Wait up to 60 seconds. SMS can be slow.
Check if your carrier is blocking shortcodes. Try texting "START" to the number if you received anything.
Make sure you haven't blocked the number.
If none of that works: Contact Member Support with your phone number and they can check if the send failed on the backend. You can reach out to our Member Success Team through the chat feature. Simply click the AlterMe icon in the bottom-right corner of your screen to start a conversation.