Gradient pattern illustrator

Hello,

I created a pattern which is simply circles that repeat over and over again. I create a rectangle with the rectangle tool and include the pattern.

By going to Edit -> Modify colors -> redefine the colors of the illustration I can modify the color of my circles.

But I would like to create a color gradient from left to right.

So I tried Edit -> Modify Colors -> Horizontal Gradient but it shows me this message: You must select at least three paths to use this filter.

Do you have a solution to correct this error message or another solution to achieve this gradient?

I am using Illustrator CC 2017.

I thank you in advance .