Domain Coexistence is a separate license for On Demand Migration Tool. If it was not purchased you will need to reach out to your Account Manager for further assistance.
If you did purchase it, please make sure the account that was used for the purchase has 'Billing Administrator' role. This can be checked under the 'Access Control'. Check KB below:
https://support.quest.com/on-demand-migration/kb/306590/how-to-use-access-control-to-add-users-and-set-their-roles-within-odm