Can't Drag Pattern to swatch panel

Hi!

I was hoping you can help me with a problem I'm having with my patterns.

I created a pattern in .AI and when I'm trying to drag it into my swatch panel, it appears an empty dash box with a white box with a plus sign. Can you guys help me fix this problem? Why my swatch panel is not taking my pattern? I'm working with latest .AI version.

Thank you!