you can only use parameters matching the type of the trigger.
So a microflow triggered from a page can only pass the available entity of that page. A microflow used as event handler only the single object of the related entity.
In you microflow; I guess the Medewerker parameter is not accepted. As far as I understand, this microflow is triggered from a page containing a single Project object or it is used as event handler at the entity Project.
If you need the Medewerker object as well in the microflow, you need to retrieve it. Either by association of Database, depending on the relation between Project and Medewerker.
Overview of available input parameters:
- Single object of same type as data view
Nested data view
- Single data view object
- Nested data view + single data view object of enclosing dataview
- Single object of same type as list(selection)
- List of objects of same type as list(selection)
- List of objects of same type as list(all pages)
Nested list widget
- equal to single list widget
- + single data view object of enclosing dataview
- Single object of entity
Some additional advise:
Parameter are the white arrow like shapes in microflows.
A Before Commit event handler only expects the object it's handling, so in this case just the 'project' object.
You could have one parameter 'project' and then retrieve the associated objects over association in the BCO event, however in your case I would not use an event handler at all. You should create a microflow that does all the logic you want it to do (that could include committing other objects) and then just commit the project-object.