Reolink PoE NVRs.
According to the screenshot you provided, it seems that you have set a password before for your IP camera when you try to view it via Reolink app/ client.
So please go to Menu-> IP channel, please input that password correctly and then click on Apply button.
After that, the camera can be added to NVR successfully and you can watch the preview.
(Please refer to the below tips):