In the "edit session" interface in a campaign, under the heading "Characters & Parties", there's the description "Select the Parties and additionnal Protagonists (PCs) you want in the session."
Should be "additional".
1. Go to a campaign
2. Create a session
3. Go to the "sessions" tab
4. Click "edit session" for the session you created
5. Scroll down until you find the "Characters & Parties" heading
6. The description with the typo is immediately under it
This was fixed alongside our recent release, so I will be closing this.