Priscilla was last seen on Thursday morning in the Queen’s Park area.
She was wearing a navy blue jacket and black trousers.
Priscilla’s cousin, Sarah B, shared an appeal via Instagram.
She said: “My family, my cousin Priscilla, who is autistic, with limited vocabulary, went missing this morning in the Queen’s Park, Kensal Rise/Kilburn area of northwest London.
"Last seen at 6:16am this morning in black joggers, navy blue long jacket and natural hair tied up, leaving home to go for a short walk.
"Although the police have been notified, we are extremely concerned for her wellbeing. Please if you can kindly help by reposting & support us to find her or have any information please contact 07841365660 / 07494431767 or DM sarahb.official on Insta."
In another post shared on Friday, she added: “The search continues...I was able to lay my head down last night, In the comfort of my home, Feeling safe and secure. For Priscilla not being able to articulate herself and her uncertainty of the situation may be the reason why a stranger is not aware that she’s missing!
“Please bear this in your consciousness that not every missing person can vocalise their fear!!”
Chanel Ambrose, another cousin of Priscilla’s, tweeted: “MISSING! My cousin Priscilla is missing, please repost and get in touch with the details on this post if you have or hear any information on her whereabouts.“
A Met police spokesperson said: “We are appealing for information about the whereabouts of 21-year-old Priscilla.
“She was last seen leaving home in Queen’s Park yesterday morning (20 May) wearing a navy blue jacket and black trousers.
“Telephone If you can help please call 101 quoting CAD 2727/20May.”