I have a portafilter espresso machine but my espresso tastes very bitter. Do you have an idea what I'm doing wrong?
There are several possible reasons why your coffee may taste bitter when brewing with your espresso machine:

1. Water temperature - The water should be heated to around 160°F (71°C) for optimal extraction of flavor from the coffee beans, but many consumers prefer slightly warmer or cooler temperatures depending on personal preference. If your water is too hot or cold it can affect how well the oils in the coffee bean are extracted. Try heating the water using the manufacturer's recommended method and see if that helps improve the quality of your brew.
2. Grind size - Espresso machines use different grind sizes dependent on the desired strength of the drink. Using a coarser grind will produce more volume, but may also extract more bitterness than necessary. Conversely, using a finer grind will yield less volume and could result in reduced flavor intensity compared to a traditional pour-over cup. It is important to experiment with various grind settings until you find one that works best for your particular machine and blend ratio.
3. Temperature of the puck - When making espresso at home, most people use a thermoblock or similar device which heats up the puck used to tamp down the grounds before pressing. However, not all espresso makers employ a heat source during this process. In order to achieve proper extraction, your puck needs to reach about 195 F (91 C). This can vary based on the specific model of machine being used, so it's worth giving some thought as to whether you're using a suitable puck or not.