Your ArcSWAT3 add‑in shows Target Version: 3.0, meaning it was compiled against ArcGIS Pro 3.0 assemblies.
You are running ArcGIS Pro 3.4.0, whose ArcGIS.Desktop.Mapping.dll has product version 13.4.0.55405, while the error message shows ArcSWAT is looking for version 13.5.0.0 of that assembly, so the required .NET DLL version is not found and Pro throws the exception. 