Springboot hit runtime error: could not resolve placeholder - spring-boot-gradle-plugin

Question

I am hitting the following error when run the springboot gradle app. May i know if there is any way for external config to reference the the variable in application.properties?
Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'default.user' in value "${default.user}"
at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:180) ~[spring-core-5.3.24.jar:5.3.24]
at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126) ~[spring-core-5.3.24.jar:5.3.24]
at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:239) ~[spring-core-5.3.24.jar:5.3.24]
at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:210) ~[spring-core-5.3.24.jar:5.3.24]
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.lambda$processProperties$0(PropertySourcesPlaceholderConfigurer.java:191) ~[spring-context-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:936) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1332) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1311) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119) ~[spring-beans-5.3.24.jar:5.3.24]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399) ~[spring-beans-5.3.24.jar:5.3.24]
... 17 common frames omitted
src/main/resources/application.properties
app.user=testaccount
src/main/resources/config/testing.properties
default.user=${app.user}
code
#Value("${default.user}") private String defaultUser;
i have tried by adding the following in the application.properties but it is still not working.
spring.config.location=/config/testing.properties
spring.config.additional-location=/config/testing.properties