I have successfully been able to copy assignments from my sandbox to another county sandbox. Some of the assignments copy successfully but others don't and I get this RED message: "Couldn't find valid settings for this link." I have looked in the CANVAS community and Google.
This is difficult to troubleshoot without seeing what you have done, and how the original assignment was structured.
I am assuming that when you say you, "copy assignments from my sandbox to another county sandbox", you mean from one Canvas account to another Canvas account, likely using an export file. Let me know if I am wrong!
The error message is telling you that the copied material has a link to content in the original course which cannot be accessed. You will need to also import whatever content your original assignment was linked to..
Beyond this, you would need to provide us a screenshot of the original assignment to be more helpful.