When I attempt to open, it says SetDisplayMode failed! I ran as administrator and legacy. Nothing has changed! Very unhappy!

Have you found a fix? I just got the game and this is the same error I'm getting.

Same here.
I had this game a year ago though, and it worked fine then.
Got a new computer yesterday, and now it doesn't work.
I've changed every possible variable and nothing worked.
Roller Coaster Tycoon 2 works fine, and it shouldn't.
It should be running into the same problem.

Hello, unfortunately I have the same problem with the SetDisplayMode error that you had. Have you been able to solve the problem in the meantime? I've already tried a few things, but even though I changed the colors to 256 under "Properties" and "Compatibility", the game still won't start.
I would be very grateful for a suggested solution.

I had this issue too but managed to fix it. For me, the issue was related to my monitor configuration. My PC is a laptop which I use as a desktop. I keep the laptop closed almost all the time and have it plugged into a Thunderbolt dock which has an external monitor, keyboard, and mouse connected to it. It started working when I unplugged the Thunderbolt cable and launched the game on the laptop directly. After that, if I plugged the dock back in and duplicated the display on both screens, it would fail again. However, if I closed the lid on the laptop and ensured Windows only showed my external monitor in the Windows display settings, it started working again, even on the external display.

I'm using Windows 11 24H2. I recommend going to Settings -> Display and making sure there is only one monitor there.

What tipped me off to try using the laptop without the dock was the log file produced by a DirectDraw compatibility layer I tried from https://github.com/narzoul/DDrawCompat/releases. I noticed in the log file that it was trying to initialize using the resolution and refresh rate of my laptop screen even though I was using my external monitor. Note that this compatibility layer wasn't needed for me to run the game once I realized what the issue was. It just helped me find the problem.

After tweaking a bit more, I managed to fix another issue where Alt-Tab wasn't working well to switch out of the game and back to it. To address that, I used the latest DDrawCompat release at the GitHub page I mentioned in my last post. Go to "Assets" in the latest release on GitHub and download the zip file without "-debug" in it. Copy ddraw.dll from inside that zip file to the "C:\Program Files (x86)\GOG Galaxy\Games\Anno 1602" folder next to 1602.exe. Then create a file named "DDrawCompat.ini" with these contents: "AltTabFix = noactivateapp". This configures the DirectDraw compatibility layer in a way that makes Alt+Tab work very quickly without any glitches. I just thought I'd share in case it helps.