"Experimental modules found: Place blocks" but I am using core 8.6
The module is present since Drupal 8.2, but since Drupal 8.4 it has been marked hidden. That explains why you get the error, but you don't find the module: The module was enabled before updating to Drupal 8.4 or later, and it is still installed; once Drupal was updated to any version after 8.4, the module has been just marked as hidden (without changing its status of installed module).

If you look at the content of the block_place.info.yml file (core/modules/block_place/block_place.info.yaml), you will notice the following lines.

package: Core (Experimental)
version: VERSION
core: 8.x
hidden: true


If you remove the hidden: true line, and visit admin/modules to install a module that you then uninstall, you will see that module listed in /admin/modules/uninstall, and you will be able to uninstall it.
Installing and uninstalling a module is done just to force Drupal to clear the cache it uses to keep the list of found modules. In this case, Drupal will notice the module is not marked anymore as hidden, and it will show it again.