Hi Sunish,
What's your detailed Windows bit and Outlook bit? Please make sure you have installed the latest updates for your Office applications. Please try Clart's suggestion to create a new Outlook profile.
Additionally, you can open the task from browser and then approve to see if the task can be completed. If there is no issue in browser and the issue only happens to Outlook, please try to repair Office applications from Control Panel > Programs and Features,
select Office, click Change > Repair (Online Repair for click-to-run version).
Regards,
Winnie Liang
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