When I attempt to turn on Manifest support, I get an error saying that the URL cannot be found:

However, the URL definitely exists as it's the most commonly visited URL on our website, showing the Latest Activity stream. 

I would assume that it would be possible to use the Latest Activity stream as the custom Start URL as this is the URL that makes the most logical sense out of any URL on the entire platform. 

What could be causing this to happen?

(I've used the exact format mentioned in the description)

 

That page does not appear to be available to guests, which is likely causing your issue

21 hours ago, Marc Stridgen said:

That page does not appear to be available to guests, which is likely causing your issue

Yes, you're correct - we're a private community so guests should absolutely not be able to see anything at all. 

How do we fix this?

Either don’t set a custom landing page or make the landing page publicly accessible. You don’t have to make the full site accessible… but remember a manifest file can be read by guests, so it needs somewhere to go. 😉

Have you tried this without entering custom?

Hi everyone. Coming back after a long pause on this feature. We finally enabled it in our community. However, we haven't been able to input the "latest activity" feed as the main page (/discover URL). 
 

Quote

That page does not appear to be available to guests, which is likely causing your issue

As I stated before this is correct, but we're a private community so guests should not be able to see anything at all. 
Is there a workaround for we can input this private page as the start URL? 
Thanks! 

There is no workaround for that unfortunately at present.