Hello, happy to help you today dear :)
It could be that your data file is too large. this may need more time to export, firstly, please make sure you have enough storage to save the exported PST file, generally, they are some steps help improve performance for Office applications, such as Windows OS performance, updates, add-ins, or incompatible device drivers and so on. Regarding this, please try to check as following:
Please confirm you have installed latest version of Office applications. (Open any application, such as Outlook, go to File > Office Account > Update Options > Update Now)
Check if there is a windows update for your OS.
Please try opening your outlook in safe mode which will help disable add-in. then export or backup email to an Outlook .pst file.
If you still fail to finish exporting, we need to collect more information for further investigation.
Is there any error message if you fail to export? If yes, please share a screenshot of the error message with us when exporting.
Please go to File > Office account to check your Outlook version
Hope this helps! have a great day
Alyssa