She is the Princess Sophia, she was married twice, the second with the brother of Queen Frederica, George, she was daughter of Princess Alice. I read that this poster was placed a little before the visit of Prince Charles in 2018. It is obvious that they selected the picture wrong.
Its time to start again with unknown for many Royal residences of buildings like this:
Queen's Amalia Tower or in greek Pyrgos Vasilissis in Ilion, Attica
The Queen’s Tower Estate was born out of Queen Amalia’s vision of the establishment of a model agricultural facility, where innovative production methods were to be applied aiming to the modernization of the greek agronomy.