R Markdown pdf_document2 PDF bold greek letters

I want to use bold Greek letters in R Markdown and modify a little example from R Markdown pdf_document2 PDF figure and table numbers by section, which reads as

---
title: "Testing Bold GreekLetters "
author: "Authors"
date: "January 2019"
output:
  bookdown::pdf_document2:
    fig_caption: yes
    latex_engine: xelatex
    number_sections: true
    toc: yes
    toc_depth: 2
editor_options:
  chunk_output_type: console
link-citations: yes
linkcolor: blue
subparagraph: yes
citecolor: blue
urlcolor: blue
header-includes:
  \usepackage{amsmath}
  \usepackage{bm}
---

# test bold greek letters

$\bm{\alpha}$ and $\bm{\beta}$

$$T^{2}=\left[\overline{\mathbf{X}}_{1}-\overline{\mathbf{X}}_{2}-\left(\boldsymbol{\mu}_{1}-\boldsymbol{\mu}_{2}\right)\right]^{\prime}\left[\left(\frac{1}{n_{1}}+\frac{1}{n_{2}}\right) \mathbf{S}_{\text {pooled }}\right]^{-1}\left[\overline{\mathbf{X}}_{1}-\overline{\mathbf{X}}_{2}-\left(\boldsymbol{\mu}_{1}-\boldsymbol{\mu}_{2}\right)\right]$$

Then errors occur as follows

! Improper alphabetic constant.
<to be read again> 
                \mitalpha 
l.93 \(\bm{\alpha}
                \) and \(\bm{\beta}\) 

How can I fix this problem? (PS: \bm in \usepackage{bm}; \boldsymbol in \usepackage{amsmath})