I would first check that you correctly have the 'nexus-startup.cfg' in your TFTP root folder, so that it's not a path issue. I suggest this because the uri error your getting would suggest the path is not found. Also - you'll need to address your syntax. Following from Nx-OS contextual help:

 tftp:  Enter URL "tftp:[//server[:port]][/path]"

Also - I would try just copying it to your flash and then to startup-config after that if it's not liking the syntax you used to copy it over.

I would first check that you correctly have the 'nexus-startup.cfg' in your TFTP root folder, so that it's not a path issue. I suggest this because the uri error your getting would suggest the path is not found. Also - you'll need to address your syntax. Following from Nx-OS contextual help:

 tftp:  Enter URL "tftp:[//server[:port]][/path]"

Also - I would try just copying it to your flash and then to startup-config after that if it's not liking the syntax you used to copy it over.

UPDATE:

This known issue with your NX-OS version could be related, or even the root cause:

CSCud05567
Symptom : Following a system switchover, copying to bootflash on the supervisor module does not work.

Conditions : This symptom might be seen under normal operating conditions for a Cisco Nexus 7000 Series device.

Workaround : None.

Source is HERE

The good news is that this issue is only appearing from 6.1, so if you are in a position to move down to 6.0 family of NX-OS you should be good. My personal preference anyway is to stay a family behind the latest and greatest unless there's a security patch.