删除旧的build.gradle文件，重新构建项目
1.无法连接代理：* What went wrong: A problem occurred configuring root project 'android-build'. > Could not resolve all artifacts for configuration ':classpath'. > Could not download gradle.jar (com.android.tools.build:gradle:3.5.0) > Could not get resource 'https://dl.google.com/dl/android/maven2/com/android/tools/build/gradle/3.5.0/gradle-3.5.0.jar'. > Could not GET 'https://dl.google.com/dl/android/maven2/com/android/tools/build/gradle/3.5.0/gradle-3.5.0.jar'. > Connect to 127.0.0.1:31181 [/127.0.0.1] failed: Connection refused: connect
代理服务器无法连接：
找到.gradle/gradle.properties文件，打开修改：
#systemProp.http.nonProxyHosts=localhost;127.*;10.*;172.16.*;172.17.*;172.18.*;172.19.*;172.20.*;172.21.*;172.22.*;172.23.*;172.24.*;172.25.*;172.26.*;172.27.*;172.28.*;172.29.*;172.30.*;172.31.*;192.168.*
#systemProp.http.proxyHost=127.0.0.1
#systemProp.http.proxyPort=31181
#systemProp.https.nonProxyHosts=localhost;127.*;10.*;172.16.*;172.17.*;172.18.*;172.19.*;172.20.*;172.21.*;172.22.*;172.23.*;172.24.*;172.25.*;172.26.*;172.27.*;172.28.*;172.29.*;172.30.*;172.31.*;192.168.*
#systemProp.https.proxyHost=127.0.0.1
#systemProp.https.proxyPort=31181
把代理设置全部用#号注释掉。
2.无法下载gradle的相关文件，设置build.gradle仓库地址为国内阿里云
buildscript {repositories {// 使用阿里云镜像maven { url 'https://maven.aliyun.com/repository/google' }maven { url 'https://maven.aliyun.com/repository/central' }google()mavenCentral()}
