If you are unable to import Office 365 mailbox into PST then With this office 365 backup to pst converter software which offers a bunch of features with no restriction on exporting in one go, you can easily import Office 365 to Outlook along with all the respective properties.
It will keep safe your original data formatting as it was before conversion. It allows you to backup Office 365 emails, contacts, and calendar to PST, MSG, EML, EMLX, and MBOX. The app gives only a few simple steps to achieve migration from Office 365 folder to the required format in any single and bulk mode just a few minutes.
Read more info:- https://www.filesrecoverytools.com/blog/how-to-convert-office-365-user-mailbox-to-shared-back-to-user-mailbox/