I am using kdenlive version 20.4.1, and am trying to make a slow motion video. However when I open the change speed menu and drag the slider, it says "Minimum speed is 100."
https://imgur.com/a/xBObAEj

what kind of clip are you using (video format ... and maybe hardware source)
have you tried to produce a new slow motion clip directly with a CLIP JOB (project bin) .. right mouse on it , clip jobs, duplicate clip with speed change

also have you tried to press CTRL on right edge of the clip and enlarge duration using mouse?

It turns out that I couldn't slow down the video clip because it was surrounded by other clips.
Thanks for the help.

The same happened to me,

I couldn't slow down the video clip because it was surrounded by other clips

when you slow down, the width of the video clip increases and won't fit in its current space,

to solve this you have to "make space" for the new width by moving, deleting or resizing the other surrounding clips