The 27-year-old injured her shoulder in training at the world championships in Colombia and was only able to finish fifth in each of her three qualifying races, failing to advance as one of the final 20 riders making it to the quarter-finals.
She had needed to finish as one of the top two racers who had not already qualified for Rio at the championships.
Walker, who finished fourth at the Beijing Games, has been plagued by injury since the last Olympics and needed surgery to fix a badly broken arm suffered while training in February.
She sustained 11 fractures to her arms in four separate crashes, one of which, at an event in the United States in 2014, also left her with a badly injured knee and concussion.
(Reporting by Greg Stutchbury; Editing by Peter Rutherford)
