Each Microsoft account comes a mailbox for both email and tasks. If your mailbox or cloud storage is full, you won’t be able to sync your Microsoft To Do tasks.
You have 5 GB of free cloud storage with your Microsoft account shared across your files and photos in OneDrive, attachments in Outlook.com and your Microsoft 365 apps. You also get 15 GB of free Outlook.com email storage which is separate from your Microsoft cloud storage.
Sign in to Outlook.com and check your email storage settings to see what’s taking up space.