Calculating Backlog

First of all:  Welcome to the exciting world of Power BI 😃


It totaly depends on the data you have.
In case there is a table containing at least the following info, it is easy:
FactTable

orderID int,
value decimal,
regionID int,
requestdate date, 
shippingdate date (empty if not delivered)

the following might help:

BacklogLastMonth = CALCULATE(SUMX(FactTable, FactTable[value]), deliverdate = BLANK(), DATEDIFF(requestdate, NOW(), MONTH) = 1)

CALCULATE is basically used to overwrite filters during calculation only.
SUMX may be switched by SUM for this purpose.
After telling to CALCULATE what to calculate, (the sum in this case), you can add filters.
First one here being deliverydate = BLANK() for a shipping not to belivered and the second condition DATEDIFF(requestdate, NOW(), MONTH) = 1 means the requestdate was within the previous month, as requested.
For more info about CALCULATE, read the doc: CALCULATE function (DAX) - DAX | Microsoft Docs