DAX Newbie Question - Base Table Expected

I know that I can define variables & reuse them in subsequent expressions to create more readable code.

I've seen others (better) do it

Keep coming across '... base table is expected'

For example -

// Using Wide World Importers.Application.StateProvinces 
DEFINE 

//Sum population of States in a Sales Territory -> Works OK
VAR _Table1 = SUMMARIZE(States,States[Territory],"Population",SUM(States[Population]))

//Find most populated Sales Territory -> Works OK
VAR MAXPOP = MAXX(_Table1,[Population])

//Put them both together = base table is expected
VAR Result1 = CALCULATETABLE(_Table1[Population],FILTER(_Table1,[Population] = MAXPOP))

//So, lets do this -> Works OK
VAR Result2 = CALCULATETABLE(SUMMARIZE(States,States[Territory],"Population",SUM(States[Population])),FILTER(_Table1,[Population] = MAXPOP))

EVALUATE Result2

DAX is quite happy

Where I would like to end up is a scalar variable = "Southeast"

DAX is happy with - (for persistent table)

DEFINE
VAR DenseState = MAX(States[Population])
VAR getState = LOOKUPVALUE(States[State Name],[Population],DenseState)
EVALUATE {getState}

 But not happy with - (virtual / cte / #temp kind of table)

VAR getTerritory = LOOKUPVALUE(_Table1[Territory],[Population],[MAXPOP])