Following is from Wikipedia
Electronegativity, symbolized as χ, is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond.1 An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus.
Whereas an element's reactivity is based more on how ready it is to gain or lose electrons. Here is a visual explanation of the situation.
As the atoms move from left to right, the electronegativity of these atoms are decreasing as the distance between the electrons and the positively charged nucleus is would be greater, giving it lower ability to attract electrons. However, as you said the reactivity series says potassium is more reactive than sodium. Let me explain...
The potassium would have a lower tendency of attracting electrons compared to sodium, it would hence more readily lose electrons to form potassium ions (K+) to form ionic bonds with other elements. The reaction would tend to occur more readily hence the reactivity of potassium is higher than sodium.
There are also other cases in the reactivity series that are explained in a similar way. Eg. Sodium and Magnesium. (For this you need to consider only the net charge of the ions formed as the charge would be considered neutral once electrons and protons are equal). The magnesium ion has charge of 2+ while Na has a charge of 1+, this would mean that the there is stronger attraction between the valence electrons of Mg than the valence electrons of Na. The magnesium will lose the electrons less readily and so it will be placed lower than sodium in the reactivity series.
The reason why it is different for the non metallic elements is that they need to gain electrons in order to react and form bonds. This would mean that a higher electronegativity would give it a higher ability to gain electrons and hence react more readily, placing them higher in the reactivity series, unlike the metallic elements that would go through a different scenario.
In summary, the electronegativity of an atom affects the reactivity whether it needs to gain or lose electrons to react. Hope this answers your question!