I am unable to claim a keep in cyrodiil because the first one on my list is of the opposing alliance. It won’t let me scroll down to choose a guild in my alliance. This has happened to several other guild members also.
Just to double check: do you have perms in those other guilds (of the correct alliance) to claim keeps? That permission is based on guild rank, and not every GM gives those perms to all ranks. If several people in your guild are experiencing the same issue, it may be due to that.