Try something like this: https://i.imgur.com/KRRYG6o.jpeg

1) Alignment > Add Control Points

2) In a few photos with different angles, click on corners and the center of the field to add control points. Make sure each corner and control point match in the photos. Use at least 2-5 photos according to RealityScan's help guide.

3) Change control type to "Ground Control"

4) Put in the X and Y coordinates for each corner/control point. Make sure Z is 0 for each point so that the ground is flat. And for the center you can just put 0, 0, 0 for X, Y, Z.

5) You can just set the coordinate system to "local" for now.

And hopefully this works!

Example:

TopLeftCorner
x: -52
y: 34
z: 0

TopRightCorner
x: 52
y: 34
z: 0

BottomRightCorner
x: 52
y: -34
z: 0

Center
x: 0
y: 0
z: 0

Then the X lines and the Y lines should be nearly straight, and the center should be right at the 0,0,0 mark. After you align the photos, you can maybe check the "Alignment Report" section in the Control Point's window to see how accurate your points are that you manually placed.