I invited a Microsoft to-do list, but the other party couldn't join

For Microsoft To Do in a work or school environment, shared lists only work between accounts within the same place of work or education. If both accounts are in the same organization and the invite still shows "can't join list" while a false join notification appears, the supported causes and actions are:

1. Confirm the list was not deleted or access was not limited
   • In To Do or Outlook on the web, open the shared list.
   • Open Sharing options or Manage access.
   • Check whether sharing was turned off, or Limit access to current members was enabled. If so, the old invitation link will no longer work.
   • If needed, stop sharing and create a new invitation link.
2. Recreate the invitation if the original invite is stale
   • If the recipient appears as already invited but cannot join, remove the existing share/invite and create a new one.
3. Verify both users are using the correct account type
   • Microsoft services sign users in based on the account type. A work or school account must be used for the business service.
   • Personal Microsoft accounts and work or school accounts do not synchronize, and using the wrong account can cause sign-in or access errors.
4. Check whether the account is supported for Microsoft To Do
   • For work or school use, Microsoft To Do requires a supported Microsoft 365 plan and an Exchange Online mailbox for storage and sync.
   • If the account is incompatible, disabled for To Do, or uses only on-premises Exchange without a cloud mailbox, the user will not be able to use To Do correctly. In that case, the IT admin must verify licensing and mailbox setup.
5. If only some users are affected, this may be a known service issue
   • Microsoft Q&A reports the same symptom: users receive "Can't join list. It looks like this list isn't available" and the owner may see a join notification even though the user is not added.
   • In the cited case, Microsoft identified it as an ongoing incident and advised admins to check the Microsoft 365 Admin Center service health for issue ID MO1213801.

If both users are in the same organization, using the correct work account, and a newly generated link still fails, the next step is for the Microsoft 365 admin to check:

• that Microsoft To Do is enabled for the affected user,
• that the user has a supported license,
• that the user has an Exchange Online mailbox,
• and whether there is an active service incident affecting shared To Do lists.

References:

• Why can't I join a shared list?
• What's the difference between a Microsoft account and a work or school account?
• Troubleshoot Microsoft To Do in Office 365
• Create and manage task lists with My Day in Outlook
• Create and share lists
• people cannot accept to do list link - Microsoft Q&A
• A recieved invitation to join is a bit old and i cant accept without the screen saying something went wrong. - Microsoft Q&A