a new camera which is active and working on the NVR
Working in what way? When there is no Live View.
Is it in the connected camera list under Live View?
when I try adding it into the view page
I'm not quite sure what you mean by that.
Cameras appear in Live View when they are successfully connected.
I don't know what you'd do to "add" them.
Going back to basics :
it only shows previous 2 and get an error message "no more IP camera allowed" when I try adding it into the view page but shows up active in the camera management
Is this an NVR with 8 PoE channels? You have not specified the full model.
If so - these are pre-assigned and fixed, such that cameras can't be '"added".
To connect the (unspecified, assumed Hikvision) camera, in the NVR web GUI camera management page, you'd need to find the IP address of the channel you want to connect the camera to.
It will likely be something like 192.168.254.x
Power up the camera while connected to the LAN and with SADP set the camera IP address to be the same.
In the NVR web GUI camera management page, selected the chosen channel, change the mode to Manual, set the known camera password, select Hikvision as the protocol, 8000 as the port and click OK.
Connect the camera to the selected NVR channel and it should connect.