You are trying to create a so-called hard-surfaced model.
Meaning flat sides.
This triangulation happens when four or more vertices are not lying on the same plane.
Which is hard to keep, or to fix when you model. This happens on purpose or by accident when you move a vertice, unknowing its relation to other parts of the model.
You can influence this effect by switching on auto smooth
and changing the angle to make it away.
Also check for common error like:
- duplicated vertices
- inside faces (use
x-ray
to see inside model.
Topology topology.
As FedPete says, odds are that face is not flat.
You might get away with selecting it in face mode and right clicking and selecting shade flat for that face. Its not perfect or ideal but on the side of a keyboard it may be good enough.
i make smart uv mapping and it fixed thanks for the help <3