Fatal Error on module install

Problem/Motivation

Attempted installing module on a local, already-existing Drupal 11 site to test out module.

Upon enabling module, receive the following error:

Fatal error: Cannot use Drupal\Core\Block\BlockPluginInterface as BlockPluginInterface because the name is already in use in /var/www/html/web/modules/contrib/subsite/subsite.module on line 9

Attempted on an already-existing local Drupal 10 site and got the same error.

Installed a brand new local Drupal 10 site to test and got the same error

Steps to reproduce

Attempted on brand new local install of Drupal 10.5.1
Added basic modules and enabled (admin_toolbar, pathauto, token)
Installed/enabled social media links module
Installed/enabled book module
Installed and then enabled the Sub Site module and got the error message:

Fatal error: Cannot use Drupal\Core\Block\BlockPluginInterface as BlockPluginInterface because the name is already in use in /var/www/html/web/modules/contrib/subsite/subsite.module on line 9

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes