SMAPI 3.8.3 says that the oldest required SDV version is 1.5.2 but the https://smapi.io/ site says that the oldest required SDV version is 1.5.1
To Reproduce

1. Install Stardew Valley 1.5.1 (I've used GOG version)
2. Install SMAPI 3.8.3
3. Try ti run the game
4. See error:
   

Thanks! I updated the site to show the correct version.