In 2.10, C2A only works if there is already an alpha channel (in 2.8, this was added automatically). So use Layer>Transparency>Add alpha channel first.
PS: by the way, C2A by definition, alters the alpha-channel, so if the alpha-channel is locked nothing will happen.
PS: by the way, C2A by definition, alters the alpha-channel, so if the alpha-channel is locked nothing will happen.