elgg_access_collections and elgg_users_sessions are tables in your database and Twitter® Bootstrap 4.1 for Elgg is an elgg plugin with files and folder. These two are no way related to each other. You cannot place files and folder in a database. You need to extract the plugin files inside the mod directory (absolute path in your hosting space)

There should be a directory with the name "mod" in the install folder of your Elgg installation. Just look into the directory on your server where you have installed Elgg into and you should find it. If you want to add a new plugin, just copy the plugin folder of the new plugin into the "mod" folder (you should see the plugin folders of the plugins that come bundled with Elgg already in the mod folder). After you have copied the new plugin folder into the mod directory, you can enable the plugin from the admin area of your Elgg site. The Plugins page in the admin area lists all available plugin and you can activate/deactivate them from there.

Thank you Rohit and iinonly for your answers and help. My problem was then that I could not enter my elgg installation to locate my mod folder. I contacted my server host and got the passwords for FTP and SFTP and also downloaded https://cyberduck.io/ in purpose to log in to the server and my elgg installation to find mod folder. And sorry that it took so long time for me to answer you.

yours

Björn