As of March 11, 2021, you won't be able to sign in to To Do on any platform if you don't have an Exchange Online account.
Microsoft To Do needs an Exchange Online mailbox to store and sync your tasks, which means on-premises mailboxes need to be moved to Exchange Online.
If you use To Do or the Tasks app in Teams with an on-premises mailbox, you’ll lose your To Do data when your on-premises mailbox is moved to Exchange Online. To prepare for that, make sure to either print your lists or save them as PDFs so you can manually recreate them in To Do.
When you tell employees you’ll be migrating their on-premises mailboxes to Exchange Online, remind anyone who uses Microsoft To Do to print their lists or save them as PDFs. People who use the Tasks app in Teams will need to download To Do so they can print or save their lists.
Once you've migrated their mailboxes to Exchange Online, employees will need to manually reenter their saved list information in To Do.
If you use Microsoft To Do or the Tasks app in Teams, your To Do lists and tasks may be lost when your organization migrates its mailboxes from on-premises to Exchange Online. As a precaution, print your To Do lists or save them as PDFs. Once your mailbox has been migrated to Exchange Online, you can manually reenter the information you saved. For more info, talk to your IT admin.