This error means that you are trying to render multiple AOVs into a single file, and the file format (PNG in this example) does not support that.
JPEG, PNG, and TIFF cannot have multiple AOVs in a single file. The AOVs have to be rendered in separate files.
If you get this error, check the Common tab. If you don’t see the <RenderPass> token, then you’re going to get the “more than one output statement was associated with single layer driver” error.
For JPEG, PNG, and TIFF, you need the <RenderPass> token:
Normally, MtoA will automatically add the <RenderPass> token for you. But sometimes Merge AOVs gets enabled for EXRs, and then the Image format is changed to another format, like PNG.
So just switch back to EXR and disable Merge AOVs. Then switch back to the other image format.