last_day() showing error in Calculated Column

Hi,

I am trying to create a Calculated Column to find out the last date of the month based on the "CREATED_AT".

For that I tried with the following formula using SQL engine.

LAST_DAY (to_date("CREATED_AT"))

Note: CREATED_AT is Nvarchar(8).

But I am getting the below error.

Can anyone suggest me why I am getting this error.

Note: This function is working absolutely fine in SQL console.

Regards,

Subrat