Why is the chart button missing when trying to create a chart using Easychart?
Have a look at the Easychart support request "Installation on Drupal8". It contains varioous hints about what might be the cause of your issue, such as in these comments in it:


nr 3: You need to install embed &amp; entity_embed to get the chart buttons.
nr 8: I ... didn't have Libraries API module installed.
nr 8: It appears to look for ec.min.js. The Easychart library that drush installed for me didn't have ec.min.js , but copying ec.full.min.js to ec.min.js makes it all work


Note: I added the hyperlink here for the Libraries API (it's the commonly used one, there is no Easychart specific variation of it ... at least not that I'm aware of).