OriginalGriff's and Abhinav S's answers point out to you the reason for the specific error you are getting.
However, I'd like to draw your attention to what appears to me a rather strange aspect of your code:
1. assuming this is a DragDrop handler.
2. your outer iterator:
foreach (Control ct in this.Controls)
Is iterating over every (top level) Control in the scope of whatever type of context it's in (Form, UserControl).
3. it appears that you then add the dragged Control to every instance of every Panel, or every GroupBox.
a. adding the same instance of a Control multiple times to the same, or to different, container Controls, is going to result in the place where the Control "is" being "in" the last container Control you added it to.
b. however, if it's the case that your Form, or UserControl, here has only one Panel, and one GroupBox on it: this could work as expected. Or, this could work, obviously, if the Form, or UserControl, contains only one Panel, or one GroupBox.
Example: if I have a Button, 'button1, a Panel, 'panel1, and a GroupBox, 'groupBox1, and execute this code:
No error is thrown at compile time, and 'button1 ends up inside 'groupBox1.