Latex error: missing delimiter (. Inserted) {\xi \left( {p,{p_q}} \right)} \right|}}

Ideal formula ,

\begin{
    equation}
{
    P_2} = \dfrac{
    {
    {
    P_2}^{
    '}}}{
    {
    \left| {
    \left( {
    p,q} \right)} \right|}} 
\end{
    equation}

Report errors ：

Missing delimiter (. inserted). ... {
    \xi \left( {
    p,{
    p_q}} \right)} \right|}}
Missing $ inserted. ... {
    \xi \left( {
    p,{
    p_q}} \right)} \right|}}
Extra }, or forgotten $. ... {
    \xi \left( {
    p,{
    p_q}} \right)} \right|}}
Extra \right. ... {
    \xi \left( {
    p,{
    p_q}} \right)} \right|}}
Missing delimiter (. inserted). ... {
    \xi \left( {
    p,{
    p_q}} \right)} \right|}}

The reason for the error is that The absolute value \left|,\right| There is an error . Solution \lvert Instead of