In some research, the SWAT models are created differently. Only the model input files and some of the shapefiles are available. In this case, you could manually create the swat project and use the SWAT viewer. The video will show you how to do that.
For the subbasin and reach shapefile, make sure the subbasin id column exist and has valid values. The SWAT viewer will use the subbasin id to search corresponding results. The column name must be “subbasin” and the type should be integer or long.