I've replaced all '\' characters with '/' in the path, but I can't see where the error is.

​@dublove try to understand what we are trying to make you understand. If your code flow involves having the path as a string constant inside the code then to answer your question, it won’t work. If it is obtained from some API call, then the code will work no change needed.

The issue is happening before your replace() even runs.
var oPath = "D:\Program Files\Adobe\Adobe Photoshop 2020\Photoshop.exe";
In that line, the \ characters are already being treated as escape sequences, so the string is not what you think it is by the time you call:

var nPath = oPath.replace(/\\/g, "\/");

So the replace won’t fix it because the backslashes have already been “lost”.

You need to define the path correctly first, either by escaping:
var oPath = "D:\\Program Files\\Adobe\\Adobe Photoshop 2020\\Photoshop.exe";
or just use forward slashes from the start:

var oPath = "D:/Program Files/Adobe/Adobe Photoshop 2020/Photoshop.exe";

At that point, you don’t need the replace at all.

You can’t “fix” a broken path string after the fact you have to define it correctly first.

You need to escape \ in the path as well. Else the code will treat \ and the character following it as an escape sequence, due to this your \ is lost. The following will work

var oPath = "D:\\Program Files\\Adobe\\Adobe Photoshop 2020\\Photoshop.exe";
var nPath = oPath.replace(/\\/g, "\/");
alert(nPath);