What should I do, as I have vowed to slaughter a sheep and invite friends, and I am now able to slaughter but cannot gather the friends I specified?

A conditional vow that depends on a disliked condition is binding, and one must fulfill it. If the intention behind it was to draw closer to Allah by feeding people, then you must fulfill it. If it is difficult to reach those whom you specified, you should wait until you can. If it becomes completely impossible, then you must offer expiation for a broken oath (kaffarat yamin). However, if you only intended to express joy and happiness at the success, and not to draw closer to Allah, then this is a permissible vow, and some scholars allow you to choose between fulfilling it or offering expiation for a broken oath.