Alternatively, you should have no trouble with a manual installation: https://codex.www.ads-software.com/Installing_WordPress

I have a website: https://www.artcarter.com (Is no a WordPress Website)

Just check source of this site, shows WordPress structure. There seems to be WordPress installed on root as well and hence the error.
Please check in you file manager in root we folder(www), you should see the wp-includes, wp-content confirming the same.
If you need to update the same installation with new site, you would need to remove the existing installation same way you installed it i.e. either manually or through Softaculous equivalent and then install the new site there at root.