Common Sudoku mistakes
Writing a digit that already sits in the same row, column, or box. That is the simple clash. Check mistakes will show it.
Leaving a stale note. A 7 still penciled in after 7 is already in the box will hide the real move. Erase notes as soon as they die.
Guessing between two candidates and hoping one works. A board with one solution does not need that. If two digits still fit, look at another cell.
Skipping the last-empty-cell scan. The obvious fill is often still there, one group over.
Undo, fix the group, continue. You can restart the same puzzle if the board has drifted. How to play Sudoku is the page to reread. Play Sudoku on Beginner if you want a quieter grid.