I am rather new to Tor and not sure if it is something I missed, or that I have done wrong. Almost every single onion site (90% of them) I try to access shows the error message "Unable to connect".
This is 100% normal. Most onion services are run on personal computers and not on enterprise servers and are likely to go down as often as people turn off their computers.
In an interview earlier this year, Roger Dingledine, one of the founders of Tor said:
"commercial companies like to inflate the size of the “dark web”, which is easy to do when they don’t define what they’re talking about. The reality is that there are only a few thousand websites available as Tor onion services currently, which makes the “iceberg” metaphor about the huge dark unknown particularly misleading."