When conversation state is 'initializing', conversation may not be updated or used for message sending until it reaches states 'active' or 'inactive'. 'Initializing' state appears in GMMS conversations when not all resources were yet created to start message sending. It's possible when Conversation with Participants endpoint is used for GMMS conversation creation or when GMMS conversation is autocreated by inbound sms message.
Participants of GMMS conversation are not yet created in background.
Wait till participants are created. To ensure that state is not 'initializing' wither fetch conversation till state changes to 'active' or 'inactive' or use onConversationStateUpdated webhook.