When I had EssentialsX installed, I ran the command: /speed 3 or something like that. A few days later, I no longer had a need for EssentialsX so I uninstalled it. After I uninstalled it, I still had the speed effect so I ran the command /effect clear @s minecraft:speed, but it gave an error like No effects found.. I still have the speed effect and I can't get rid of it.

Speed applies an attribute to the user, not a potion effect. The only way to remove it is to install Essentials again and do /speed 1. There really isn't anything Essentials can do for you besides that.