You need everything set up per all the information linked from https://help.mypurecloud.com/articles/about-sms-messaging/
In particular you need either an SMS-capable phone number through Genesys Cloud Voice or to have the 3rd-party Twilio integration set up.
Also, of particular importance for outbound, you need the proper paperwork submitted to allow sending outbound SMS messages as explained in the SMS Regulations section. Otherwise all your outbound messages are blocked.
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George Ganahl GCP, GCSME, ICCE, ICHD, etc.
Senior Principal PS Consultant
Genesys
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