Changing the default data file in outlook 2019 error

[ThanksForHelping]

My default data file account is set to outlook account but when I want to set the gmail account, I get the following error "The selected outlook data file (.pst) format cannot be used. Outlook data files (.pst) must use the same format as .....". 

I have already the followed the method in https://docs.microsoft.com/en-us/answers/questions/69776/outlook-2019-default-data-file-error-pstost...

I tried removing the data file account, but it doesn't allow me because it's set as the default account and I get the following error - "The default data file cannot be removed.". 

The plan was to remove the data file and then add again with unchecking the "use cached option" from the control panel.

Any solution for change the default data file would be appreciated. Thanks. 

[#2]

@ThanksForHelping if your account is an outlook.com account, you cannot set it to online mode. 

I do not recommend an IMAP account as the default data file - your calendar and contacts will be in a folder labeled 'this computer only' and at risk of being lost as they are not backed up anywhere. 

If you want the IMAP email address to be default, it's not a problem (and is not related to the default data file). You can set the start up folder to be the IMAP inbox. (File > Options > Advanced - at the top.) 

[#3]

I have already set my default email to gmail but I'm more interested in setting up the data file - any ways?