You are using Bubble’s own chart plugin here, not amCharts. This thread is for a different tool.
You are using :count which will not work with any type of chart. :count counts the number of items in a list to produce a single number - charts will need a list of numbers to work.
There are good examples in the thread that I shared previously (here) of how to use Bubble’s own charts with the :group by… feature. If you are still not able to work it out then I would repost your issue in that thread.
Ed