SMAPI 4.5.2 is the loader that Stardew Valley 1.6.14 and later needs before any mod can run. Install it once per game installation: download the release, run the platform installer, and let it add the launch option for Steam or GOG.
The SMAPI console opening beside the game proves the install worked. Keep one mod folder, update SMAPI after each major patch, and remove a mod by deleting its whole folder rather than individual files.