Rename Layer 1 as Background Layer, and select the first keyframe in the Timeline.
Click the Oval tool on the left, SHIFT drag to draw a circle of any size on the canvas.
In the Transform panel where you can set the size and position of the circle, set its size to 400*400 and the position is 150(X), 70(Y), which is the center of the Canvas as you can see.
Go to the Shape panel. First set the Line Width to "0", and choose Radical in the Fill Style.
Move the Gradient pointers to the position indicated in the picture below.
Set the color of the first pointer
as R:146 G: 251 B:255 A: 100%.
Set the color of the second pointer as R:27 G:77 B:178 A: 0%.
The effect of the background layer is illustrated as below: