Follow this guide to figure out where you have GTA 5 Legacy installed: TUTORIAL: Correct GTA V Installation Location(s) & IVPath Fix
After that copy the file path:
Then press WINDOWS + R,
Type %localappdata%/FiveM/FiveM.app, then click “OK”
Locate the File CitizenFX.ini and open it.
This will open a text file. Look for the line that starts with IVPath=.
Then paste the file path you coped to IVPath=your file path here.
Then save the file after and try running FiveM.