DAX error : A single value for column 'Date' in table 'Date' cannot be determined.

Hi tvaishnav 

Iteration functions requires a table as first argument. VALUES is a function that creats a table contains the unique values in a column. You cannot refer the column directly. 
Iteration functions creat a filter context over the tablw under iteration. Only the columns that belongs to this table can be refrenced directly. Columns from related tables in the "one" side of the relationship can be referenced using RELATED function. The "many" side related table can be referenced using RELATEDTABLE functions which bring a set of rows from the "many" side table which are realted to the current row on the iteration. In this case an aggregation must be provided in order to evaluate any column from this related table. 
on the other hand CALCULATE function can be used to perform context transition in irder to evaluate columns from rekated tables. In this case aggregation shall be provided. Remember that any refrenced measure is automatically wrapped with CALCULATE. 
please let if you still have any doubts