If you have 50+ rings, but also have a shield, you will not be able to transform into Super Sonic. You have to lose the shield via a damage source first.
Save State: https://cdn.discordapp.com/attachments/315258983399161866/382930821381881856/CantSSBugged.gsx
In his Super Sonic Blasts into Sonic 3D - Director's Cut Part 2 video, at about 2:14 he explains this is intentional. Here's the video link: https://youtu.be/iFbki9Y-r_4?t=2m14s