1) Open a browser to https://portal.office.com
2) Click on your profile image at the top right, then the Sign out button. 

3) Close Teams and all browser windows. 
4) Open a new browser to https://portal.office.com and login as you normally would. 
5) Open this link to verify you have access to the conferences SharePoint documents. Conferences - Documents - All Documents (sharepoint.com)
If you get an unauthorized message, email help@healthtech.com and ask for permissions to the Conferences SharePoint document library.
6) Re-open Teams and try opening the Conferences Team, General channel, SharePoint tab again.