How to create a column that repeat the previous value until a condition is fulfilled?

Hi,

This M code works

let
    Source = Excel.CurrentWorkbook(){[Name="Data"]}[Content],
    #"Trimmed Text" = Table.TransformColumns(Source,{{"Beach", Text.Trim, type text}}),
    #"Added Custom" = Table.AddColumn(#"Trimmed Text", "Custom", each if [Quality]="null" then [Beach] else null),
    #"Filled Down" = Table.FillDown(#"Added Custom",{"Custom"}),
    #"Filtered Rows" = Table.SelectRows(#"Filled Down", each ([Quality] = "Good")),
    #"Reordered Columns" = Table.ReorderColumns(#"Filtered Rows",{"Custom", "Beach", "Quality"}),
    #"Renamed Columns" = Table.RenameColumns(#"Reordered Columns",{{"Custom", "City"}})
in
    #"Renamed Columns"

Hope this helps.