@waleed111 - There are three primary kinds of things that you can create in Power BI
Columns and Measures formulas need to return single value (a scalar) while tables formulas need to return a table of values. When you are having problems with DAX formulas like this, it is always good to check the documentation:
A table with the same number of rows as the table specified as the first argument. The returned table has one column for each pair of <name>, <scalar_expression> arguments, and each expression is evaluated in the context of a row from the specified <table> argument.
Therefore, for the column formula, you will need to aggregate the table or perform some calculation that returns a scalar value, like wrapping that statement with a COUNTROWS function or one of the X iterators like SUMX, AVERAGEX, etc.