First of all, make sure you have the latest version of Geometry Dash and Geode. Then, check if Geode has installed properly.
On Windows, go to your Geometry Dash folder, and check if the following files exist: Geode.dll, GeodeUninstaller.exe, the geode folder, xinput1_4.dll. You can also check if your antivirus has quarantined Geode itself. Don't worry, this is a false positive, and you can add an exception for the detected file.
For Android, make sure you also have the latest version of the launcher.
If you are trying to install Geode on Linux, go here.
If all else fails, you can join the discord server and open a new help thread.