looks like npm does not have permissions to modify the file system (or has create but not modify permissions) from the account running it.
Did you install node/node-red as and administrator and now running is as a non admin user?
I would check that the account running node-red has the necessary permissions for that folder c:\users\xxxxx\.node-red\ and its sub directories .
If you need to get moving, you could try stopping node-red, opening an elevated command prompt, CD to that c:\users\xxxxx\.node-red\ and perform the install manually npm uninstall node-red-contrib-whatever
As far I know, there is no permissions issue.
This happening because even after deleting the nodes some file Ex.: serialpot.node or bindings.node is being used by node-red process.
Its will work in below scenarios:
First:
Install node-red-contrib-modbus node from palette manager
Simply Try to uninstall the node from palette manager without using it
Second:
Install node-red-contrib-modbus node from palette manager
Drag and Connect to any server from TCP/Serial and Deploy the node-red
Delete the node and configuration node and Deploy the node-red
