If the process starts when a platform event message is received,
associate the process with a platform event and an object, and specify
matching conditions. Because every process acts on a Salesforce
record, it requires a single record as a starting point. That way, the
criteria and actions know where to start evaluating and executing.The process fails if it finds:
1) Multiple records that match the criteria.
2) No records that match the criteria.
This bit is bit funny from SF end, But you can bypass it, but providing a dummy record.
Create a dummy Object, with an external field `D_External__c'. Then create a single record of that dummy Object. And set Value of External Field as constant. eg 78234564654
Now add an extra field in your event
My_External_Dummy_Field__C , and default its value as
78234564654 . The one you defined in the record you created.
This will make sure, each event fired will have My_External_Dummy_Field__C as
In your Process Builder, Match your Dummy_Object's external field with
As this matching condition will always match because of single record of dummy object and setting the default value of the platform event, your process builder will always run. And you can create do whatever you want.