A measure is calculated over dimension values. If used without dimensions then a measure returns only one value.
So if you want to use a measure as a dimension you probably want to use the measure values calculated for each of the dimension values of a dimension. So you need to use the aggr function to build a virtual table with the list of measures per dimension value.
Something like this:
If( avg(Time) < 20, 'X<20',
DimensionA, DimensionB, ...etc )