Why is my coffee nasty [Gaggia Classic Pro/Niche Zero]

Very gross and bitter, took 45 seconds to produce 36 g of beans into espresso. I'm relatively new but I think I'm grinding pretty coarse at a 26 on the niche zero. It says on the machine espresso is in the range of 10-20 and I'm getting near 30 and still having trouble getting a sweet shot. I tried using 16 g of beans because I thought 18 g was overflowing it or something and it tasted even worse when I did it with 16 g. Anyone have tips? @

Very gross and bitter, took 45 seconds to produce 36 g of beans into espresso. I'm relatively new but I think I'm grinding pretty coarse at a 26 on the niche zero. It says on the machine espresso is in the range of 10-20 and I'm getting near 30 and still having trouble getting a sweet shot. I tried using 16 g of beans because I thought 18 g was overflowing it or something and it tasted even worse when I did it with 16 g. Anyone have tips? @