How to solve “Can’t complete your request” error while accessing shared mailbox in Outlook webmail?

Step 1: Sign into Office 365 admin portal via https://portal.office.com

Step 2: Click on Admin from the left pane and navigate to Groups >> Shared mailboxes.

Step 3: Click on the shared mailbox you cannot access.

Step 4: Scroll down to Manage mailbox permissions section and click Manage permissions.

Step 5: Respective to the permission click on Edit.

Step 6: Click on Add permissions and check the checkbox respective to your account and click Save.

Step 7: Remove and add the shared mailbox folder in Outlook.