postgis update multipolygon with st_makevalid() gives error

ST_CollectionExtract will pull out a particular type of component from a GeometryCollection. For example, ST_CollectionExtract(geom, 3) will return a Polygon or MultiPolygon.

Since your column type is MultiPolygon, you may need to wrap ST_CollectionExtract with yet another function, ST_Multi, to coerce Polygon geometries into single-component MultiPolygons.

Beware ST_MakeValid - it generally does what you'd expect it to, but it's not magic and it can occasionally "correct" your geometry in unexpected ways. It's a good idea to run a SELECT query first, to see what it has in mind.

You need to update your geometry column by extracting Multipolygons from GeometryCollection. Simply run below query and you are good to go

update table set geomcol = st_multi(st_collectionextract(st_makevalid(geomcol),3))
    where st_isvalid(geomcol) = false;