Return N/A value

This is a bit annoying, I wish the card had a setting for alternate text to return when the result is blank. But you can do this in a measure by explicitly checking for the blank value

eg

Prev Months Tickets = VAR CurrentMonth = MONTH(TODAY())

VAR currntYear =  YEAR(TODAY())

VAR LastDay = EOMONTH(TODAY()),0)

VAR Firstday = DATE(CurrentYear, CurrentMonth,1)

VAR ContextTable = 

CALCULATETABLE(

      VALUES(Dates[Date]),

      Filter(Dates,

                Dates[Date] >= Firstday &&

                Dates[Date] <= LastDay))

var result = 

CALCUALTE([Total Tickets],

DATEADD(ContextTable, -1, MONTH))

RETURN IF( ISBLANK( result ), "N/A", result)

You could also look at using the COALESCE function for the last line

RETURN COALESCE( result , "N/A" )