Here's a VIDEO that may help you- Enable SMS feature for employee or you can also use the following instructions to solve the issue.
1. Go to the Employee section, search the Employee name with the help of the funnel icon.
2. After searching the Employee, at the top right there is a pencil icon available to edit the employee info.
3. On the Employee detail section, in the Employee biography subsection at the bottom, click the checkbox "Enable SMS", and click the "Save" button to enable the texting feature. Click "Close" to exit from this section.