Hello @nipunadv,
Since your date fields are in two separate tables. You can try out this formula but there’s an condition. In my file I’ve created a common field known as “ID” and based on that I created a “One-to-One relationship” between these two tables with “Bi-Directional” filter. Below is the screenshot provided for the reference of the data model -
After this I created a “Calculated Column” in the Table 1 i.e. Order Date table. Below is the formula provided for the reference -
Dates Difference =
DATEDIFF( 'Order Date - 1'[Order Date] , RELATED( 'Request Date'[Request Date] ) , DAY )
So now the end result looks like this. Below is the screenshot provided for the reference -
I’m also attaching the working of the Excel as well as PBIX file for the reference.
Hoping you find this useful and meets your requirements which you’ve mentioned above. 
Thanks and Warm Regards,
Harsh
Dates - Harsh.xlsx (9.6 KB)
DATEDIFF Function - Harsh.pbix (22.1 KB)