How do I insert a block into a node?
The insert block module implements an "input filter" via the core Filter module. A text format is a collection of enabled input filters. So when enabling a module that provides an input filter, the next step would be to configure the text format to use the input filter.

The Full HTML text format does not have that input filter enabled by default.

In addition the module has a Drupal 8 development release, but the code underneath still seems to use PSR-0 for its class loading and has not been updated in 3 years. It may be broken for other reasons as well.