It sounds like you are running a very old version of snapd. Could you run snap version and include the output here?
The first thing I’d try is to install the available operating system updates. That will likely allow snapd to correctly bootstrap. You might also want to consider using a newer version of Ubuntu: 16.04 finished its 5 years of regular support back in April.
Have you tried installing the available operating system updates? As I said above, I suspect you’re running version originally released in 16.04 back in 2016, rather than the update released earlier this year. If you install the updates, things might start working.