Whats problem in this formula is invaild ?
IF NOT Override Shipping Method? OR IS BLANK (Override Method).THEN+Shipping Method ELSE Override Method
ISBLANK is one word. Incidentally, that formula should be applied to "Final Shipping Method", not Override Method. Override method is something the planner will enter. No formula needed there.
I am still facing error what to do ?
You also have a period "." after the ISBLANK(Override Method).
Your formula is correct but you just have to clean up the extra characters.
IF NOT Override Shipping Method? OR ISBLANK(Override Method) THEN Shipping Method ELSE Override Method