@saz12 the main cause of 'Outlook Data file cannot be accessed' is problem in PST file. So, if you have created the new PST then, you need to create this file as default for Outlook account. When the PST file is created, again go to account settings and then you will find the option to change folder. Select the new PST file and subfolder to store messages. Close all windows and then click on Send/ Receive.
Restart Outlook and the error will be resolved. One important thing to remember is that Outlook should not be connected with exchange server during the process..You can also use Outlook PST repair tool to repair PST file and fix common errors