Hi Justyna Kobyka, thanks for your post.
When accessing SharePoint on Office 365, do you have the option to click on the Action Gear (Settings) > Site Settings > Site Collection Administrators, is your account also listed here? If not, go ahead and add your account.
Next, try accessing the Action Gear (Settings) > Site Contents > ArcGIS Maps > App Configuration to configure the app. Let us know if you have any issues!
Thanks,
Rudy