If panel_textbox_kelimeekle_yab is already a textbox on your form, then you don’t need to declare its type when you assign a value to it. C# thinks you’re trying to declare a new string variable.
Change that line of code to
panel_textbox_kelimeekle_yab.Text = Ingkelimeler[(Ingkelimeler.Length+1)];
This probably won’t solve all your problems, but at least it will get you on to the next error message. (You probably mean Length-1 in your array index, but there’s really no way for us to know.)