Solder won't stick to metal tab, no matter what I do
If solder doesn't stick to a metal surface, I see three possible reasons for this:

The part you want to solder is not hot enough - the metal surface has to be brought to the melting point of the solder, otherwise it will not make sufficient contact and the solder joint will eventually fail. If that is the case: get more heat.

An oxide layer keeps the solder from making contact - in this case you need to remove the oxide layer by either scratching it off or using flux to break up the oxide layer. Depending on the type of metal you might need a special flux. E.g. if you want to solder to aluminum, flux intended for use with copper doesn't work.

The solder is not soluble in the metal you try to solder to - solder and metal will form a quasi-alloy at the junction, but this requires solubility. There's nothing you can do if solubility is not given, just like you can't dissolve sugar in oil.


So, not all metals can be soldered together. For example you won't find any solder that has sufficient solubility in both stainless steel and copper. Some metals - like aluminum - can be soldered to copper in general. But doing so is tricky: aluminum forms a strong oxide layer very fast. So after scratching it away you have to protect the solder point from oxygen (e.g. coat with oil) until and especially while soldering (oxide formation is much accelerated at high temperatures). This is all a big pain in my experience.
Another important point is the galvanic series: if two metals have different nobility, the less noble metal will be corroded away in the long term. That's the reason why aluminum and copper should not be soldered together, though it is possible.
Take a look at this effect for aluminum and mercury.