Appsheet sms error: "to" missing or invalid

[JenniferJero]

I want to make a bot that sends an sms automatically to several people on a specific date.

When I use my own phone number in 'to' field, it works.

번호=phone number

But, when I use column in 'to' field, I'm getting an error.

This one:

Error encountered in step with name [Send a notification]: Error: No SMS was created because 'To' is missing or invalid

I list all country codes US, KR. 

I paid for twilio and appsheet.

I don't know what to do further.

Any suggestions or tips on how to handle & resolve my issue would be greatly appreciated.