Those pages check three things before opening and redirect you if any is missing:

1. Onboarding not finished – if your sending domain's Validate Link step has not been completed, you are sent to /admin/marketingcampaigns/onboarding. Finish all three onboarding steps.
2. No plan selected – if no email plan has been chosen yet, you are sent to /admin/marketingcampaigns/billing. Pick a plan (the Free plan works).
3. Monthly allowance used up (Campaigns and Flows pages only) – if the emails you already sent this calendar month have reached your plan's limit, you are also sent to Billing so you can upgrade.

Super-admin accounts skip these checks. Note that the Templates and Tracking pages only require a plan, not completed onboarding.