The value entered violates data validation rules set in cell E6

The value entered violates data validation rules set in cell E6

Hi, I was trying to shift a data from sheet 1 to auto paste into sheet 2 by press from row and colume at = 'e6' from page 1 and press enter then there have an error pop up said:  The value entered violates data validation rules set in cell E6.

Please advice what is the solution and what is going on?