Good afternoon,
If you have a screenshot of your Terrain in the inspector view it might help us to
get a bit more context to provide a more accurate answer.
Did you managed to get your character on your scene without this issue previously ?
If yes : what are the changes since the last revision ?
If not :
Have you checked that the terrain is not flipped (rotation of +/-180 for x or z ) ?
Is the Terrain Collider present ? Enabled (idem for all parents game objects until the root of your scene) ?
Is the problem still present if you disable all of your scripts (e.g. is one scripts disabling the gameobject containing the terrain) ?
Is your character bellow the terrain at the beginning of your scene ?
Otherwise you can also try to copy / paste an example from the hands-on about locomotion
onto a blank scene to have a clear comparison between your scene with the issue and the
example to find the difference and fix your issue.
Stay safe, good luck and Happy Easter,
Mathias