The "claptrap" that Windows regurgitates about nobody having permission to do anything is a generic message that is displayed whenever a local PC has a problem accessing a remote PC. It is unhelpful and usually inaccurate.
If the remote PC has firewall software installed, make sure it has been configured to allow access to the other PC(s) in the network. This is normally done by adding their IP addresses to the "trusted" area.
Make sure that you have at least one folder set as "shared" in each PC, by right clicking and selecting the Sharing & Security option.
Try accessing the remote PC by opening My Network Places and entering it directly into the Address Bar, either by its Name or its IP address, for example:
Another option is to map a network drive. From the Tools menu, select the option to "Map Network Drive" and enter the path to a shared folder, for example:
If you normally log in to the remote computer with a Password, use the option to "Connect using a different user name" to enter the Username and Password that you log into the other computer with.