Stuck on R-number
This isn’t an error message, but a lot of photographers come across the camera being stuck on an r-number with it refusing to take a photo when the shutter is fully pressed. This could be anything, such as r36 or any other number. What it actually shows is the remaining number of photos that can be taken on the buffer and appears when you half-press the shutter.
So why won’t your camera take a photo when it is fully pressed, but instead still shows the r-number message? Well there are a few reasons for this. By default your camera will prioritise the focus of an image before taking a photo (this can be overridden in the settings), so if you are shooting in a very dark environment and your camera can’t obtain proper focus then you’ll see this r-number until it locks on to something.
Other reasons include having the camera set to the self-timer or a remote of sorts; the r-number will show until the timer is complete or the remote is triggered.