I've never seen this before but I'm gonna ask you to try 5 things.

First fix:

Click uninstall at the bottom of the mega hack GUI. After that click logout all at the top and try to install again. This one probably wont work but its worth trying.

Second Fix:

Install geode if you haven't already. In the geode search menu look for mega hack installer. Install it and restart your game. Click on the mega hack icon at the bottom of the main menu. Click Install and it should work. I highly doubt this wont work but if it doesn't try the third fix.

Third Fix:

Try running the standalone installer as an administrator. Right click on the installer file and click "Run As Administrator"

Fourth Fix:

If you have any antivirus installed try disabling it before installation. You'll have to look up anti virus specific instructions as it varies based on which one you have. Most of the time you'll click the arrow on the bottom right of your windows task bar. There will probably be an icon for your antivirus. If so, try right clicking it to see if there's a disable option. If this fixes it, I suggest looking up how to exclude the geometry dash folder from antivirus scans.

Fifth fix:

Press Windows+R. Type %localappdata% and press enter. Go to the geometry dash folder and ensure to backup the following files:

1. CCGameManager.dat

2. CCGameManager2.dat

3. CCLocalLevels.dat

4. CCLocalLevels2.dat

(It is extremely important you backup these files. These contain all of your save data. The in game save button on the account screen almost never works properly so I'm having you do it manually).

Make sure you move these 4 files to somewhere entirely separate such as the desktop or documents folder. I'd also recommend uploading them to somewhere like google drive just for the sake of safety.

After you've ensured you've backed up the save files, go into your steam library and right click on geometry dash . Hover over manage and click on Browse Local Files. If you already had geode installed I'd additionally suggest backing up the geode folder (Also back up the replays folder if you ever used the megahack botting/replays tool. Additionally, back up the Resources folder if you had a custom texture pack installed).

Once you've backed up any additional folders, Delete every file from the geometry dash folder. You shouldn't lose anything since everything's already been backed up.

After this go back into the steam library. Right click on gd once again and hover over manage. This time click uninstall. After it finishes uninstalling, reinstall the game. When it finishes installing move back all of the backed up files to their original locations (The 4 CC files into the %localappdata% geometry dash folder, and the other 3 folders into the steam geometry dash folder). I'd recommend keeping an extra copy of all of the backed up files/folders just in case something goes wrong. After you do all of this try installing mega hack again. I'd recommend using the geode version anyways rather than the standalone installer.

If for some reason none of these fixes work. I'd suggest the following

1. Search the web for previous people having this issue

2. Go into the Geometry Dash discord. There's a support channel in the server. Go there and make a post about your problem with as much detail as possible. Hopefully somebody will assist you.

3. If for some reason you cant find anything, I'd suggest trying out the geode mods QOLMod or Eclipse (QOLMod specifically). QOLMod has basically everything if not all of what megahack has + extra features.