WebGL build error (unity2021.3.6)

Hi,
I used websoket.I can't build a webgl program in unity2021.3.6, it reports an error. There is no such error in unity2020.

Building Library\Bee\artifacts\WebGL\build\debug_WebGL_wasm\build.js failed with output:
error: undefined symbol: sendfile (referenced by top-level compiled C/C++ code)
warning: Link with `-s LLD_REPORT_UNDEFINED` to get more information on undefined symbols
warning: To disable errors for undefined symbols use `-s ERROR_ON_UNDEFINED_SYMBOLS=0`
warning: _sendfile may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library

https://forum.unity.com/threads/webgl-build-error-unity2021-3-6.1320369/