What is the best solder you can get in Europe with all of the regulations

I’ve been trying to solder components but oxides keep forming and I’ve been trying everything to stop this. I’ve tried using flux injecting it or using a pen, I keep adding more and more but oxides are everywhere on the pin, wire and soldering iron tip. I keep cleaning the tip, I’ve even bought more clean ones on Amazon but I keep finding black stuff on the iron every time I try to solder.

I thought maybe my solder is the problem. I’m in Switzerland which is very regulated so I only can seem to get lead-free solder especially on Amazon. I got this solder here: https://www.amazon.de/Lead-free-Solder-Soldering-Electronic-Electric/dp/B0DBSZQG44/ref=mp_s_a_1_4?crid=3U5WV03Y5I9H2&dib=eyJ2IjoiMSJ9.wTbb7Wv0KEp5T1tXIXvqkOxS0TFz_b6A3z-p7s8G94S-wZCowNfplKyB9gkkoC-1sK0he0E-Hpf8gABKJArLeRPsF9RXQEK_zmcVQ8RkXezHBN-M7hXTb7mbi1c0CWjGjvgMEz0rax30AArK21EwgGxG90vwDPi_V_t4xUNWpEMNT1180eBxNPQgy0oERERKAYo7GW7QwMZROagj1Y39ZA.g663f91Dh4cT0QO1rn4qVDIBd6Rc_yOTTCIAiLOD8_k&dib_tag=se&keywords=lead+free+solder+0.6mm&qid=1771184857&sprefix=lead+free+solder+0.6mm%2Caps%2C84&sr=8-4

I thought maybe this is the problem. Maybe the solder sucks. Does anyone know where to get good solder if you’re in Europe?