The 26-year-old had been on trial with the Pilgrims since the beginning of the month, but Sheridan has claimed that he will now explore other options.
"I'm not going to be doing anything with Ryan," he told the club's official website.
"He's not really what I'm looking for. I've explained it to him the best way I can, and I'm working hard to try and fill that area."
Dickson began his career with Plymouth in 2005.