I just wanna know if this manga panel is just an error because in Itachi Shinden novels, there was really no confirmation of the real score between Itachi and Izumi.
Hard to say. Depends on how you define 'girlfriend-kind' of love.
If the love you're talking about is the kind of love wherein he would choose her over the world, then no. If it's the kind of love where it meant choosing the former between preventing another war, even if it means having to kill her; or being with her no matter what, then yes.
In Itachi Shinden: Book of Dark Night, the following can be read:
I decided that Izumi would be first … By killing her himself, he wiped away the last of his hesitation—the hesitation borne of unfinished business with the clan, of bonds of emotion.
Itachi was hesitant. The way I interpret it, Itachi does indeed consider Izumi as a part of his life since he thought of Izumi as a hurdle to overcome. He placed her in a Tsukuyomi where she lived to an old age together with Itachi, having and raising a family together before she died, both in dream and real life. If he had no feelings for her, he'd easily kill her but he went to such lengths to give her a peaceful and painful death, with consideration of her own feelings towards him.
With this, I think it can be said that Itachi, at least, considers her as someone important to his life. Whether he truly loved her or not in a romantic kind of way would probably depend on how you define love.
They were not girlfriend and boyfriend but they were, however, seen by their peers as a couple.