Getting  Error:
Executing the 'AccountManager' method failed. Either the service isn't configured with the correct urlMapping, is not global, does not have the proper method name or does not return the requested account and all of its contacts.

@RestResource(urlMapping='/Accounts/*/contacts')
global class AccountManager {
    @HttpGet
    global static Account getAccount() {
        RestRequest req = RestContext.request;
        String accId = req.requestURI.substringBetween('Accounts/','/contacts');
        Account acc = [SELECT Id, Name, (SELECT Id, Name FROM Contacts) 
                       FROM Account WHERE Id = :accId];
        return acc;
    }
}

Test class

@isTest 
private class AccountManagerTest{
    static testMethod void testMethod1(){
        Account objAccount = new Account(Name = 'test Account');
        insert objAccount;
        Contact objContact = new Contact(LastName = 'test Contact',
                                         AccountId = objAccount.Id);
        insert objContact;
        Id recordId = objAccount.Id;
        RestRequest request = new RestRequest();
        request.requestUri =
            'https://mindful-otter-12kfkn-dev-ed.lightning.force.com/services/apexrest/Accounts/'
            + recordId +'/contacts';
        request.httpMethod = 'GET';
        RestContext.request = request;
        // Call the method to test
        Account thisAccount = AccountManager.getAccount();
        // Verify results
        System.assert(thisAccount!= null);
        System.assertEquals('test Account', thisAccount.Name);
    }
}

Getting  Error:
Executing the 'AccountManager' method failed. Either the service isn't configured with the correct urlMapping, is not global, does not have the proper method name or does not return the requested account and all of its contacts.

@RestResource(urlMapping='/Accounts/*/contacts')
global class AccountManager {
    @HttpGet
    global static Account getAccount() {
        RestRequest req = RestContext.request;
        String accId = req.requestURI.substringBetween('Accounts/','/contacts');
        Account acc = [SELECT Id, Name, (SELECT Id, Name FROM Contacts) 
                       FROM Account WHERE Id = :accId];
        return acc;
    }
}

Test class

@isTest 
private class AccountManagerTest{
    static testMethod void testMethod1(){
        Account objAccount = new Account(Name = 'test Account');
        insert objAccount;
        Contact objContact = new Contact(LastName = 'test Contact',
                                         AccountId = objAccount.Id);
        insert objContact;
        Id recordId = objAccount.Id;
        RestRequest request = new RestRequest();
        request.requestUri =
            'https://mindful-otter-12kfkn-dev-ed.lightning.force.com/services/apexrest/Accounts/'
            + recordId +'/contacts';
        request.httpMethod = 'GET';
        RestContext.request = request;
        // Call the method to test
        Account thisAccount = AccountManager.getAccount();
        // Verify results
        System.assert(thisAccount!= null);
        System.assertEquals('test Account', thisAccount.Name);
    }
}