Lookup value if date is between two dates

I don't think you've told us all about the model... I suspect there are relationships between the two tables based on the date fields.

Try this

CreatedInSprint =
var __date = WorkItems[fields_SystemCreatedDate]
return
	MAXX(
		FILTER(
			Sprints;
			AND(
		    	Sprints[attributes_startDate] <= __date,
		    	__date <= Sprints[attributes_finishDate]
		    )
		),
		Sprints[SprintNo]
	)

This should work correctly on the assumption that there is always at most one sprint returned by the logical condition in FILTER. If there happen to be many, then the maximum SprintNo will be returned.

Best

Darek