Plugins are built and maintained by their individual developers. Looking at the Grammarly plugin page in the Figma Community, it appears that it’s not currently supported in Figma Slides, which is likely why you can’t access it.
If Grammarly support for Slides is important to your workflow, I’d recommend reaching out directly to Grammarly’s support team to request this feature:https://support.grammarly.com/hc/en-us.