Cannot choose a specific custom object as the target for 'Create a Record' Global Action

Why This Limitation Exists

In Salesforce, a detail object in a Master-Detail relationship always requires a master record to exist before a detail record can be created. A Global Action operates outside the context of any specific record, so there is no master record available when the action is triggered.
If a user attempts to create a detail record via a Global Action where the Master-Detail field is not populated, Salesforce returns an error: "Invalid Data. Review all error messages below to correct your data. You must enter a value for [MasterDetailFieldName]."
Additionally, even if the Global Action is configured, users can only create the detail record if a related master record already exists and is available in the lookup search dialog. If the master record does not exist, the user would need to navigate away, create the master record, and then return to the action — which is a poor user experience.

Workaround

It is technically possible to create a Global Action for a detail object before the Master-Detail relationship is established. However, this is not advisable due to the error and user experience issues described above.

Important Considerations

* Users may encounter the error above when the Master-Detail field is not applied to the Global Action page layout or is not populated.
* Quick create function on all lookup fields (creating a master record from within a lookup) is not available for all lookup dialogs in Salesforce.
* Consider promoting the Salesforce Idea to allow Global Actions on Master-Detail child objects (see Additional Resources).