The artboard is 360 by 360 pixels, export as PNG at 72 ppi and Facebook says the image is too small, please use another...
Export the same file at 300 ppi and Facebook blows up image and it doesn't fit in the cover photo space, not even close... a picture is below.
I don't even know what to google to figure out what is happening...
You should make your cover photo the same dimensions that Facebook recommends at 820 W x 312 H pixels. You can export this at 72 dpi.
https://www.facebook.com/help/125379114252045
Your second attempt at saving as 300 dpi is too large, because you uploaded a square image into a rectangle space. The image will fill the space until the image is 820 pixels wide. This will cut the top and bottom out.
