Bm

• How to Fix “Extended mathchar used as mathchar” with \bm
  The error usually means the legacy bm package conflicts with unicode-math. Remove bm and use unicode-math commands such as \symbfit.
• How to Bold Math Symbols in LaTeX
  Use \mathbf for upright Latin letters, \boldsymbol for bold math symbols, or the bm package for reliable bolding of Greek letters and operators.