Hi there, chef_reenalee92-.
Good to see you here!
I understand that you want to send the invoice via SMS. While this option is unavailable, you'll want to send it by email instead. Then, you can notify your clients through a text message that you forwarded the invoice already.
For more details, you'll want to check this link: Create invoices in QuickBooks Self-Employed.
Comment below if you have more questions. Stay safe!