Composer install the requested PHP extension pcntl is missing from your system
解決方法：
在composer.json中增加C
"config": {
"preferred-install": "dist",
"sort-packages": true,
"optimize-autoloader": true,
"platform": {
"ext-pcntl": "7.3",//7.3php 版本號
"ext-posix": "7.3"//7.3 php 版本號
}
},