The answer to your question depends heavily on what you mean by "ruler" and also on what you mean by "literacy".

Especially the second part. What you mean by literacy changed over time. Especially in the early middle ages, most of the Romance and Germanic dialects spoken in day-to-day life, even the prestige dialects used by the high nobility, largely didn't have written standards. As late as the 840s CE, reading and writing were largely considered separate skill sets that weren't always learned at the same time. It's possible to learn more or less passively to read, but writing well, not just in a compositional sense, but purely in the sense of muscle memory, requires spending and enormous number of hours practicing what is basically an unproductive and even downright anti-social activity.

As a followup, could it have been the severely-disabled Charles II of Spain? Given the contemporary descriptions of him and his massive intellectual disabilities, is there any evidence pointing to his potential illiteracy - if through no fault of his own? I've seen unsourced comments that he could "barely read or write" but they're not as well-sourced as other contemporary accounts.