You won't be able to fix this with "Save As..." because there is no way to control the geometry type in that dialog. It only lets you enter Data Source Creation Options(-dsco) and Layer Creation Options(-lco). You will have to either use the DBManager or ogr2ogr command line.
I get this error when trying to mix geometry types when importing data to a spatialite db.
The only option I see in the DBManager is the checkbox for Create single-part geometries instead of multi-part. You can try toggling this or you may need to fallback to ogr2ogr commandline tool.
So the solution is to tell it to force all geoms to a single geometry type, either POLYGON or MULTIPOLYGON. You might want MULTIPOLYGON so that the existing multipolygons are retained.
Using ogr2ogr the option that does this is -nlt where PROMOTE_TO_MULTI looks promising and I've used it with success in the past:
-nlt type: Define the geometry type for the created layer. One of NONE, GEOMETRY, POINT, LINESTRING, POLYGON, GEOMETRYCOLLECTION,
MULTIPOINT, MULTIPOLYGON or MULTILINESTRING. Add "25D" to the name to
get 2.5D versions. Starting with GDAL 1.10, PROMOTE_TO_MULTI can be
used to automatically promote layers that mix polygon or multipolygons
to multipolygons, and layers that mix linestrings or multilinestrings
to multilinestrings. Can be usefull when converting shapefiles to
PostGIS (and other target drivers) that implements strict checks for