Spring Data JPA Unable to locate Attribute with the given name

The same problem was when i had deal with Spring Data Specifications (https://www.baeldung.com/rest-api-search-language-spring-data-specifications)

Initial piece of code was:

private Specification<Project> checkCriteriaByProjectNumberLike(projectNumber: String) {
    (root, query, criteriaBuilder) -> criteriaBuilder.like(root.get("project_number"), "%" + projectNumber)
}

The problem was in root.get("project_number"). Inside the method, I had to put the field name as in the model (projectNumber), but I sent the field name as in the database (project_number).

That is, the final correct decision was:

private Specification<Project> checkCriteriaByProjectNumberLike(projectNumber: String) {
    (root, query, criteriaBuilder) -> criteriaBuilder.like(root.get("projectNumber"), "%" + projectNumber)
}

Try changing private String FirstName,LastName,Email; to private String firstName,lastName,email;

It should work.

findByFirstName in SubscriberRepository tries to find a field firstName by convention which is not there.

Further reference on how properties inside the entities are traversed https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.query-methods.query-property-expressions

As per specification , the property names should start with small case.

...The resolution algorithm starts with interpreting the entire part (AddressZipCode) as the property and checks the domain class for a property with that name (uncapitalized)....

It will try to find a property with uncapitalized name. So use firstName instead of FristName and etc..

After I change my entity class variables from capital letter to small letter for instance Username to username the method Users findByUsername(String username); is working for me now .