I'm still fairly new to blender and the last few days I had been following sculpting tutorials and was able to mirror objects with no problem. But today when I was trying to mirror them it is not working. I also get this pop in the bottom saying "failed to set value". I tried to look up how to fix this but did not get the answer I was looking for and I have even updated to the latest version and I still cannot get the mirroring to work anymore. Any suggestions on figuring this out would be awesome. Thank you so much.
Your view is showing $Y$ and $Z$ axis (Green and Blue lines across the screen respectively), but the modifier is set to mirror only on $X$, which would be perpendicular to this view, therefore impossible to see.
You can add mirror in $Y$ on the modifier.
It helps if you use a different view, that clearly shows the axis you want to work on (like top view)