Cannot change/select/Add older .PST file from Account Settings n O2016

[Hixpix]

Hi,

Installed Outlook 2016 on brand new HP PC running under window 10 x64. I was able to add a 2010.pst file to 'my' email account such that I can get both the new and the old data sources in the left hand pane of Outlook but could not do the same for my wife's account.

I have copied the old.PST file to the same folder as her new Data File.
I select <Account Settings\Data Files\Open File Locations>
The Explorer App launches to select the file.
Then when I select the file I receive an alert warning saying

"A dialogue box is open, close it and try again"

I cannot find an open dialogue box.

Going round in circles now, any help would be appreciated.

Thanks



Hixpix