Expression.Error: We cannot apply field access to the type Number.

It isn't in the current context. Try the following syntax:

        Table.TransformColumns(
            RowData, 
            {
                {
                    "Value", 
                    each _ * Number.RandomBetween(0.95,1.05),
                    type number
                    }
                }
            )

However, note that this isn't doing what you think. Put this in a new custom column:

Number.RandomBetween(0.5,.95)

Every value is the same random number. You can either try the method in this article, or do what I would do. Either use a Measure in DAX, or a calculated column in DAX where each row or iteration is really random.

MS should document this in their  Number.Random* documentation, but they don't. I just avoid both Number.Random and Number.RandomBetween in Power Query. DAX is better here, usually.