Welcome to the forum!
When you click "Export As" button, you have to put the full path, including the file name, there. So for example C:\Users\brynn\Pictures\Inkscape\mandalas\man16.png. Or else whatever it is for your operating system. If you only put the file name, it won't work.
If you click "Export As" button, it allows you to choose the directory. Just browse to the place where you want to put it, type the file name, and click Save button.
(If you don't put your own file path there, and let Inkscape decide where to put it, Inkscape defaults to the directory you see in Inkscape Preferences > System > Temporary Files)
Then you can't forget, after you have the whole file path, you still need to click the Export button, in the Export PNG dialog. (That happens to me a lot
)
Oops, there's Moini, sneaking in while I type