We don't currently support iCloud accounts that have 2FA enabled. However, we plan to have support for this ready in the next week or so. Please keep checking back with us periodically to find out when this is ready.
In the Security section, click on "Edit" to expand the Security details. In the "Two-Factor Authentication" subsection, look for the link to "Turn Off Two-Factor Authentication". When the confirmation dialog appears, click "Turn Off Two-Factor Authentication" again.