Hello,
Are you using a local PST/OST file or an Exchange/Outlook.com account?
Close Outlook and restart Windows.
Use File > Tools > Mailbox Cleanup to find large or old items.
Delete or move large attachments outside Outlook.
Archive old emails via Clean Up Old Items.
Compact PST/OST: Account Settings > Data Files > Settings > Compact Now.
Ensure Outlook and Windows updates are installed.
If problems continue, run the Inbox Repair Tool (scanpst.exe) on your data file. Please let me know how it goes. Thanks, and keep safe.