Convert the geometry to WKT, the text format the Sites and Assets import expects for lines and polygons. Shapefiles can go straight into custom tables, but assets need the WKT step first, done with QGIS and a spreadsheet.
Video: mWater Converting Shapefiles on YouTube
This is the step that stops most GIS imports. Point data loads happily as latitude and longitude, so people reasonably assume a pipe network will too, then find the importer will not accept a shapefile for an asset. Knowing that lines and polygons need WKT, and that the conversion is a short QGIS job, turns a blocked import into a ten minute task.