Returning a result with 1 DAX function and 1 Column

e0318476 

maybe you can try this

Measure = 
VAR tbl=ADDCOLUMNS(SUMMARIZE('Table','Table'[Full Name],"module complete",sum('Table'[modules completed]),"module total",sum('Table'[Total Modules])),"percentage of completion",DIVIDE([module complete],[module total]))
return CALCULATE(DISTINCTCOUNT('Table'[Full Name]),FILTER(tbl,[percentage of completion]=1))

Try one of these:

SUMX (
    VALUES ( Table1[Full Name] ),
    IF ( [Percentage of Completion] = 1, 1 )
)

COUNTROWS (
    FILTER (
        VALUES ( Table1[Full Name] ),
        [Percentage of Completion] = 1
    )
)