type User struct {
    Name     string `json:"name"`
    LastName string `json:"lastname"`
}

func (u *User) toMap() map[string]interface{} {
    return map[string]interface{
        "1": u.Name,
        "2": u.LastName,
    }
}

func HandleLambda(event events.SQSEvent) {
    userRecords := make([]map[string]interface, 0, len(event.Records))

    for _, record := range event.Records {
        ev, err := event.NewEvent(record.Body)
        if err != nil {
            continue
        }

        user, err := repository.NewUser(ev)
        if err != nil {
            continue
        }

        userRecords = append(userRecords, user.toMap())
    }

    uJSON, _ := json.Marshal(userRecords)
    // I send this to external service
}

type User struct {
    Name     string `json:"name"`
    LastName string `json:"lastname"`
}

func (u *User) toMap() map[string]interface{} {
    return map[string]interface{}{
        "1": u.Name,
        "2": u.LastName,
    }
}

func HandleLambda(event events.SQSEvent) {
    userRecords := make([]map[string]interface{}, 0, len(event.Records))

    for _, record := range event.Records {
        ev, err := event.NewEvent(record.Body)
        if err != nil {
            continue
        }

        user, err := repository.NewUser(ev)
        if err != nil {
            continue
        }

        userRecords = append(userRecords, user.toMap())
    }

    uJSON, _ := json.Marshal(userRecords)
    // I send this to external service
}

• for idx := range event.Records might as well be for idx, record := event.Records, no need have an extra line to get the actual item. And if you're never using it, for _, record := event.Records even.
• Errors are ignored? Consider logging them if don't error our immediately.
• user := make(map ...) I'd usually make that a literal to avoid repeating myself even more.
• userRecords could already be pre-allocated to the right size, then there'd be no need for append, or if you're using continue like here, pre-allocate the maximum length so it definitely won't have to be resized. Usually also doesn't matter that much if you don't have a lot of entries, but it's good to keep in mind for situations where it does matter.
• Also consider making the to-map conversion a new method on User, or a helper function, you might want to test it or reuse in other places.
• repository looks like a global variable, that's usually not the best idea. Consider making that a parameter for HandleLambda, or making HandleLambda a method and put the repository as one of its members. It will help with testability in the end to not have dependencies scattered around like that.

• for idx := range event.Records might as well be for idx, record := event.Records, no need have an extra line to get the actual item. And if you're never using it, for _, record := event.Records even.
• Errors are ignored? Consider logging them if don't error our immediately.
• user := make(map ...) I'd usually make that a literal to avoid repeating myself even more.
• userRecords could perhaps already be pre-allocated to the right size. Then there'd be no need for append at all. Or, like here, if you still need to use continue, pre-allocating the maximum length it could take, so it definitely wouldn't have to be resized might also be a good idea. Usually this also doesn't matter that much if you don't have a lot of entries, but it's good to keep in mind for situations where it does matter.
• Also consider making the to-map conversion a new method on User, or a helper function, you might want to test it or reuse in other places.
• repository looks like a global variable, that's usually not the best idea. Consider making that a parameter for HandleLambda, or making HandleLambda a method and put the repository as one of its members. It will help with testability in the end to not have dependencies scattered around like that.