One of the frequent error you have discovered in Outlook is "cannot open default emails in Outlook (.pst)". and you have to fix that in order to access your data and to fix that we have to know the causes behind this error.
There are multiple reasons that user have to face this issue
-> improper installation
-> corrupted Outlook files
-> running outlook in compatibility mode
to fix this issue or by an PST Repair tool you can go through this blog: https://www.datarecovo.c...ail-folders-outlook-pst/