SUMMARIZE returning errors (cannot convert to scalar value)

The SUMMARIZE function returns a table, but measures must return scalar values.  That's why you get the error.

Here's a measure to get the number of offices with more than 10 employees

Offices >10 Employees = 
VAR _SelectedNumEmployees = 10
VAR _Result = 
COUNTROWS(
    FILTER(
        VALUES('Employee Directory'[Office - Assigned]),
        CALCULATE(COUNT('Employee Directory'[Employee ID])) > _SelectedNumEmployees
    )
)
RETURN
    _Result

You could replace CALCULATE(COUNT('Employee Directory'[Employee ID])) in the above with a measure that counts number of employees.