My website on frontend works okay.
this happen on Hikashop admin product page.
I'm not sure when this happen exactly, because not often I am doing free download.
Before latest update was working perfect.
You can try test: create new product, upload file, set a price, Publish and come back, delete price and try set free download.
If this will working, means something maybe went wrong on my database after I deleted some product yesterday.
I'm not freak out, because in few weeks I will changing my server and core for higher version, means maybe I have to start from scratch because I couldn't use migration.
But was be good to know is not Hikashop bug.