Hi Kelly_Wen ,
Apologies for any inconvenience caused.
Your first image is weird. The dropdown should show several options:

Please check your browser's console and paste any error you see there.
Regarding the fields searched when no column is selected, it's searching in the post title, content and excerpt because those are the default fields a WordPress ListTable searches in. That doesn't make sense in this case (Purchase Orders Pro list), and we're working to change it for the next plugin's version.
Best Regards,
José Andrés