Try to purchase ticket on delta.com if you are flying from SDQ and you will get this response:
Only travel originating in Austria, Belgium, Canada, Denmark, France, Germany, Greece, Ireland, Italy, Netherlands, Spain, Sweden, Switzerland, United Kingdom, or the United States (including Puerto Rico and the U.S. Virgin Islands) can be booked via delta.com.
Only travel originating in Austria, Belgium, Canada, Denmark, France, Germany, Greece, Ireland, Italy, Netherlands, Spain, Sweden, Switzerland, United Kingdom, or the United States (including Puerto Rico and the U.S. Virgin Islands) can be booked via delta.com.