Lookupvalue between Start Date and End Date

this syntax should work:

Department =
VAR __RelevantEmployeeRows =
    FILTER (
        'Employees',
        'Employees'[Username] = 'Printer'[Username]
            && 'Employees'[Start Date] <= 'Printer'[Date]
            && 'Employees'[End Date] >= 'Printer'[Date]
    )
VAR __RelevantDepartments =
    DISTINCT (
        SELECTCOLUMNS ( __RelevantEmployeeRows, "Departments", 'Employees'[Department] )
    )
RETURN
    CONCATENATEX ( __RelevantDepartments, [Departments], "," )

the code assumes that there always is not null end date for each row in the Employees table

I'd also suggest to remove the many:many relationship in favour of a separate table with unique Usernames and 1:many, many2many relationships are not what most users think they are AND quite often if they have Both filtering direction are causing the relationship ambiguity - see detailed explanation here:
https://sqlbits.com/Sessions/Event18/Different_types_of_many-to-many_relationships_in_Power_BI