Could not get unknown property 'mavenUser' for Credentials [username: null]

I've got this error : Could not get unknown property 'mavenUser' for Credentials [username: null] of type org.gradle.internal.credentials.DefaultPasswordCredentials_Decorated during the gradle build, although I've already created a gradle properties file with the right credentials in the gradle home directory

buildscript {
        ext {
            nexusBaseUrl = 'http://....'
            // Dependencies version
            springBootVersion = '2.7.2'
            sonarqubeVersion = '3.3'
            mybatisSpringBootStarterTestVersion = '2.2.2'
            mybatisSpringBootStarterVersion = '2.2.2'
        }
        repositories {
            maven {
                url "${nexusBaseUrl}/mha-maven-public/"
                allowInsecureProtocol = true
                credentials {
                    username "${mavenUser}"
                    password "${mavenPassword}"
                }
            }
        }
        dependencies {
            classpath "org.springframework.boot:spring-boot-gradle-plugin:$springBootVersion"
            classpath "org.sonarsource.scanner.gradle:sonarqube-gradle-plugin:$sonarqubeVersion"
        }
    }