Users running High Sierra are unable to create new sites in Local.
The general issue appears to be related to not being able to connect to MySQL.
This was first reported in this Community Forum post:
A workaround does exist:
  1. While creating a new site that is hanging
  2. Create a second, throw-away site
  3. By creating the second site, the first site is able to finish creating, and the second site is able to be deleted without any issues.