Ruthie's bedroom door has a bolt-lock above the door handle, which can be seen when Annie is knocking. Therefore, there was no need to use the blue chair to keep the door closed while Ruthie wrote in her diary.
Annie questions Ruthie about why the chair was in front of her door, then walks away. If there was nothing she wanted from Ruthie, there was no reason for her to go to Ruthie's room to find out her door was locked.