In your conditional statement, you’re assigning space to s[ i ] (operator =). You want to compare them (operator ==).
Try
if (s[ i ] == ' ')
s[ i ] = ' ' is always true because the result of an assignment is the value assigned (space). This value is implicitly converted to a bool (0 = false, anything else = true). Since a space is 32 in ASCII, it will always be true.
References – Assignment Operator, Comparison Operators, ASCII Table
solved The array index doesnt show the character [closed]