Calculating difference between two column values in matrix

Curtw01 

Please try

MoM Variance =
VAR CurrentValue = [No. of Orders]
VAR CuurentYearMonth =
MAX ( 'Date'[YearMonthNum] )
VAR PreviousValue =
CALCULATE (
[No. of Orders],
'Date'[YearMonthNum] = CuurentYearMonth - 1,
ALL ( 'Date' )
)
RETURN
IF (
NOT ISBLANK ( PreviousValue ),
DIVIDE ( CurrentValue - PreviousValue, CurrentValue )
)

Curtw01 
You should be using a YearMonthNum column which is supposed to be a sequential number such as a dense rank. For example for Jan. 2023 you cannot minus one from 202301 to obtain 202212. Rather you will obtain 202300 which basically has no value in your date table. Therefore, the YearMonthNum is different than the YearMonth column which can start from 1 for example to refer to the very first month in your table and keeps adding 1 for each month after that. You can create such column using

RANKX ( 
'Date',
'Date'[YearMonth],, -- In the form of YYYYMM like 202307
ASC,

Dense
)