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MPM(Maputo) is only served by QR. You couldn't use QR as the head (first) carrier with the online tool (online tool restriction, not fare rule).
Your itinerary should start with MPM-DOH, but you couldn't make this by the online tool. You have to call an airline ticketing office or a travel agent.
By the way, MPM is no longer a good staring point of One World Explorer fare in terms of fare price.