See the askubuntu.com's StackExchange same question Unable to install "<PACKAGE>": snap "<PACKAGE>" has "install-snap" change in progress for an excellent solution!

The very top answer there, shows you how to abort the ongoing "install-snap" change for spotify, by

1. runing snap changes so see a list of ongoing changes

   $ snap changes
   ...
   123  Doing   2018-04-28T10:40:11Z  -  Install "spotify" snap
   ...
   

2. running sudo snap abort 123 to kill that running change operation.

Then you can install spotify with sudo snap install spotify without the error.