Why don't mined blocks show up in my inventory on my Spigot server?
When starting a new server, there is a property in server.properties called spawn-protection. On a standard server, the only users that can modify blocks within this region are ops.
You can make yourself an op by running this command on the server op playername. Alternatively, you can edit the ops.txt file directly and add your username to the list.
For more information about server properties, see here.