Struggling with TOPN

Yes, it was a bit hard to tell if it was working with just the one row of test data. I think the following is closer, but potentially you could still end up with multiple sectors with the same score as I'm not sure how you would want to break the ties.

Column = CONCATENATEX(
    var _topn =
    TOPN(1, ALL(Testing_Keyword[Sector]),
        var _currentSector = Testing_Keyword[Sector]
        var _sectorKeywords = CALCULATETABLE(GROUPBY(Testing_Keyword, Testing_Keyword[Weight], Testing_Keyword[Keyword]), TREATAS( {_currentSector}, Testing_Keyword[Sector] ))
        var _score = SUMX(
            _sectorKeywords,
            Testing_Keyword[Weight]  * 
            (LEN(LOWER(CRO_Company[Description])) - LEN(SUBSTITUTE(LOWER(CRO_Company[Description]),LOWER(Testing_Keyword[Keyword]),""))) /
            LEN(Testing_Keyword[Keyword])
            )
        return _score
    )
    // if the _topn variable contains all the rows from the keyword table this probably
    // means that none of the keywords matched so they all scored 0 so we should 
    // filter them all out
    var _condition = if(COUNTROWS(all(Testing_Keyword[Sector])) = COUNTROWS(_topn),FALSE(), True())
    return filter( _topn, _condition)
,[Sector]
,",")

While I was testing this I also built the following column so I could see the weighted score per sector, this may be helpful if you want to do further debugging yourself.

Column 2 = 
CONCATENATEX( ALL(Testing_Keyword[Sector]),
var _currentSector = Testing_Keyword[Sector]
var _sectorKeywords = CALCULATETABLE(GROUPBY(Testing_Keyword, Testing_Keyword[Weight], Testing_Keyword[Keyword]), TREATAS( {_currentSector}, Testing_Keyword[Sector] ))
var _score = SUMX(
        _sectorKeywords,
        Testing_Keyword[Weight]  * 
        (LEN(LOWER(CRO_Company[Description])) - LEN(SUBSTITUTE(LOWER(CRO_Company[Description]),LOWER(Testing_Keyword[Keyword]),""))) /
        LEN(Testing_Keyword[Keyword])
        )
        return _currentSector & " (" & _score & ") "
)