Conversion failed when converting the nvarchar value ... to data type int

You are trying to concatenate a string and an integer.
You need to cast @ID as a string.
try:

SET @sql=@sql+' AND Emp_Id_Pk=' + CAST(@ID AS NVARCHAR(10))

Try Using

CONVERT(nvarchar(10),@ID)

This is similar to cast but is less expensive(in terms of time consumed)