Calculate function not working

The issue with your CALCULATE function returning blank values is likely due to a problem with the PREVIOUSMONTH function or missing data in the previous month.

Possible Causes & Fixes
Check if the 'Interest Rates' Table Has a Proper Date Column

Ensure that 'Interest Rates'[Date] is recognized as a Date column, not text.
If it's stored as text, convert it using DATEVALUE().
Ensure There Is Data for the Previous Month

If there is no data in the previous month, PREVIOUSMONTH() will return an empty result, leading to BLANK().
Test by checking:
EVALUATE PREVIOUSMONTH('Interest Rates'[Date])
If it returns nothing, you may need to adjust the logic.
Use a Date Table Instead of 'Interest Rates'[Date]

If 'Interest Rates'[Date] is incomplete (e.g., missing some months), use a separate date table connected to your data.
Replace 'Interest Rates'[Date] with 'DateTable'[Date] in your formula.
Alternative Fix Using LOOKUPVALUE for More Control

Instead of PREVIOUSMONTH(), try:
Measure =
VAR PrevMonth = EOMONTH(MAX('Interest Rates'[Date]), -1)
RETURN
CALCULATE(
AVERAGE('Interest Rates'[30 year rate]),
'Interest Rates'[Date] = PrevMonth
)
This manually selects the previous month's data.