Hi Renu Abraham,
Greetings
Thank you for posting in Microsoft Community forum.
Based on the information you provided I understand you have a user who wants to disable encryption in their Outlook. Please correct me if I am wrong.
Open Outlook > Tools> Trust Center> E-mail Security> Encrypted e-mail> uncheck Encrypt contents and attachments for outgoing messages check box> click OK. Now try to send emails and check if it helps.
If you enabled the Encrypt button in Outlook on the web, disable it by running the Set-IRMConfiguration cmdlet with the SimplifiedClientAccessEnabled parameter. Connect to Exchange Online PowerShell and run Set-IRMConfiguration -SimplifiedClientAccessEnabled $false.
If the user enabled Encrypt-Only feature in Outlook, please see this Microsoft article for more information and please pay attention to the warning written in yellow before making any changes. Disabling the Encrypt-Only feature in Outlook - Office 365 | Microsoft Docs
Let us know the result so we can assist you further. Your kind understanding is highly appreciated.
Kind regards,
Simbarashe