Try hooking an Ethernet cable to it, rebooting, and then trying to run an update to see if it picks up.

If your Beam is hooked to Ethernet make sure you haven’t disabled WiFi (needed to connect to the Sub) there.

Maybe see what happens if you temporarily cable the Sub-Mini to your router and then follow the links in ‘Settings/System’ in the App to finish getting any updates and registering your device. 

Thanks everyone. I have connected the sub mini with Ethernet re-tried everything but still getting the same errors 

@advanm,
If you first power off the Sub Mini - then check the iOS Apple App Store to see if there is any App updates? If not, just manually check the Sonos App in ‘Settings/System/System Updates/Check for Updates’ 

If all is then up-to-date, reboot the router and your mobile controller device too and power on the Sub-Mini and give the setup one more try (whilst temporarily wired to router). If you do have another mobile controller device, then give that a try aswell.

If no joy after that, rather than going all-out for a factory reset of the Sub at this stage, it maybe best to submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to resolve the matter.

Thanks all. As a last ditch effort I turned everything off. Connected my Beam and Sub Mini by Ethernet 

When booted up I saw that the Sub mini was updating. But then alas, the same issue of not being able to register.

I then did a hard reset of the Sub mini and the same thing again. I’ll give the Sonos Support a call now during the week.

exactly the same thing is happening to me - did you get anything useful from Sonos support?

If you’ve already tried the things mentioned In the thread @Richard_131, it maybe best to reproduce the issue and then immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to resolve the matter, just in case yours is perhaps a different issue, plus if it is the same matter it will ensure Sonos are made aware of any issue too.

I have this exact issue with the Sub I bought yesterday. Called Sonos and they are going to escalate to teach support next week. Very frustrating.

To teach them what? Maybe it will help some others here with their issue.

I received the following suggested process from TECH support and it resolved my issue.

First, factory reset the Sub and move it next to your router for a better signal.

Second, reset the Sonos App and connect to the existing system again.

Third, follow the prompts to add the Sub back to the system. This time it should be able to complete the registration process.

Hi All,

I contacted Sonos support and we figured out the issue. 
 

Basically the PIN and serial number labelled on my Sub Mini does not match what the software believes it to be, hence it won’t accept it when registering it. 
 

Sonos will be sending me a replacement.  
 

The strange thing is that it

had been working perfectly for months so I wonder what had caused the issue?

Hey, this just happened to me recently and indid manage to fix it . Without knowing my phone was connected to a 5ghz network and not the one that used by sonos beam a 2.4 one. I connected to the 2.4 → factory reset sub mini → sub dissapears from system → initiate again the setup by adding a new product . This time it went straight without any issues. Hope it helps .

Sonos speakers actually store the exact MAC address of the original WiFi access point they’re connected to, and expect all new products to be registered on the same one. In my instance, I had added a new access point after setting up my initial Sonos devices. So when I purchased a Sub, I got that error. 
 

I fixed the problem similar to another answer here by disconnecting all but one WiFi access point and connecting all speakers to the network, and then I was able to register the sub and connect it to my Sonos Arc soundbar (after a factory reset of the sub to clear out network settings).
 

Even though your access points may have the same WiFi SSID, Sonos sees each one as a different network.