If you had Win 10 Home installed and activated then you can re-install this a often as necessary, but only Win 10 Home and I think this is your problem. You do not have an activation for Win 10 Pro which is why you are getting this error message.
Got to the MS site and download the ISO of Win 1o Home, and this time make sure it is the Home version ! This is the link -
You then need to burn this to either a DVD, if you have an optical drive, or to a blank memory stick. If you use a memory stick you will need to use something like Rufus, a free download, to convert the ISO to usable files ( https://rufus.akeo.ie/ ). For the DVD you can use either the burn ISO to disc feature in Windows Explorer or something like ImageBurn, again to convert the ISO to usable files. You may need to go into your BIOS to change the boot order to either DVD first or USB stick first, whichever you decide to use.
Then boot the computer with either the disc or the memory stick. I believe you have an option in Win 10 to do an OS install without affecting your files and applications. If you are offered this choice, take it BUT it is always a good idea to do a full backup of your data before starting an operation like this.
Any questions, post back.