Red wedding says choose a lord and lady, "if able". So if there isn't a lord and a lady on field then the effect cannot be used.

A city besieged says choose and discard 1 location with printed cost x or lower "if able". So does this mean that unless my opponent and I have a location that can be discarded I cannot use the effect of a city besieged

I thought that the term "if able" meant that unless all the effect can be resolved then you can use that effect.

Your interpretation of the "if able" on Red Wedding is incorrect. If you do not have a Lord and a Lady available, the "if able" still allows you to move to the "choose and kill one" part of the plot. It's just that if you weren't able to choose 2 characters earlier, that "choose and kill one" part fails. The plot still initiates if you don't have both. (It's passive; it has to.) It's just that both are necessary for there to be any meaningful resolution, "if able" or not.

With City Besieged, the "if able" does the same thing: allows you to choose as many targets as possible, even if you can't choose ALL specified targets. The difference is that City Besieged does allow for meaningful resolution if fewer than "all" targets are chosen. Red Wedding does not.

If both characters are necessary for red wedding to resolve why doesn't there have to be two location for city besieged to resolve.

You said city besieged does allow for meaningful resolution while red wedding does not. Why? Is there a difference in the wording or is it just a ruling from ffg

There's a difference in the wording. City Besieged says (more or less) to "choose and discard 1 location for each player, if able." If you can't get 1 location each, you can still get as many locations as possible.

But Red Wedding is considerably more complex. First, you choose a Lord and a Lady, if able. THEN you choose one of "those" characters to die. The other claims power. If you don't get a full set for the first choice (1 Lord and 1 Lady), the you can't choose one of "those" characters for the second choice. The second choice assumes and requires the full set from the first choice.

Red Wedding is a very involved, atypical card. It is probably not the best way to understand the use of "if able," particularly if you only look at the end result.

I think I understand thanks

By the way, leaving the whole Red Wedding thing aside, "if able" is best explained as doing the following two things:

1) When used in conjunction with the word "choose," it allows you to go ahead with as much of the effect as possible, even though all the things you are supposed to "choose" aren't there. (i.e., normally, you need all specified targets in order to trigger an effect; "if able" lets you trigger it even if not all specified targets are available.)

2) When use in conjunction with the word "then," it allows you to initiate the part after the word "then," even if not everything before it was successful. (i.e., normally, the part before the "then" must be successful before you can initiate the part after it; "if able" tends to let you consider the part before "then" as being successful, no matter how much of it actually happened.)