Bitterness is overextraction, which involves tweaking either grind size (extraction speed) and/or brew time (extraction duration), and of course whether you press the Espro, and consume from the vessel (spent grind still impacting flavor) versus pouring brew into a separate mug.
edit: It should also be mentioned, since AP is a relatively unconventional approach to coffee prep, it has evolved over time that many prefer to brew in the AP at significantly lower temperatures than other methods. AP instructions even suggest a 176F brew temperature, which when followed makes it more challenging to get bitter overextractions, when grind size and brew time are maintained. Consistency is a major attribute to AP.
edit: It should also be mentioned, since AP is a relatively unconventional approach to coffee prep, it has evolved over time that many prefer to brew in the AP at significantly lower temperatures than other methods. AP instructions even suggest a 176F brew temperature, which when followed makes it more challenging to get bitter overextractions, when grind size and brew time are maintained. Consistency is a major attribute to AP.