Post your formula code to check the details..
Let me clear you the first thing...
If you get any fatal errors while running the report (e.g., function returned without value,no value etc.,) the report will show
REP-0619: You cannot run without a layout.
So you just correct the function 'no_daysformula' .
First of all you run the report without that formula column.
If it works fine then , Check the return value of your formula column (Your formula column properties --> Return value --> value (It will be DATE as i think so).
As function will always return a single value, Check your formula 'no_daysformula' returns the same.
declare a return variable say for example..
RETURN V_DATE := (RETURN VALUE)
Last but not least ... use Else condition to return (NULL or any value ) in your code and check..
If any Problem persists let me know