Based on your schematic and the ECO image, I would guess that the error is due to the lack of a net name for your GND symbol. In other words, your GND symbol has a blank net name. The schematic compiler will accept that and not display any compile errors, but the PCB tool will not.
While in the schematic tool, double click on one of your GND symbols and check to see if the NET name in the properties section is set to GND. It cannot be blank (which is what I assume it currently is.) If you don't want to display the net name on the schematic, you can uncheck the "show net name field" on the right if desired, but you need to make sure the net name is "GND" or whatever you wish to label the net as. The problem appears to be related to all of your GND symbols, so you'll need to correct them all.
You can use Edit > Find Similar Objects option to select and modify them all. Method: Click Find Similar objects. Click on one of the incorrect symbols. In the dialog box, select "same" for both the Object Kind, Power Object Style, and the Object Specific Text field (which is likely blank). Verify that Clear Existing, Select Matching, and Run Inspector are checked on the bottom and hit ok. It should select only the GND symbols with the blank net, and the SCH Inspector should pop up on the side. Make sure only the affected GND symbols are selected, then just change the Text field in the inspector to say GND and the changes will be applied to the selected objects.
At that point, you should be able to import your schematic into the PCB document without the above error.
Good luck.