I dont understand properly how List.Generate() works

Anonymous The issue is at the third argument of List.Generate, when incrementing you need to consider that the current iterated value is a record so you need to use each [x= [x]+1 ]

Complete code:

let
    my_list = 
        List.Generate (
            () => [ x = 1 ],                                                                                                                                                                                                                                                                                                                      
            each [x] < 101,
            each [ x = [x] + 1 ],
            each Number.Round ( Number.RandomBetween ( 1, 101 ), 0 )
        )
in
    my_list

If there isn't any other logic that you are going to use in List.Generate then you can also try List.Trasnform

= 
List.Transform ( 
    { 1.. 100 }, 
    each Number.Round ( Number.RandomBetween ( 1, 101 ), 0 ) 
)