error while saving the value in DB, I can able to get the value from DB(one column I have manually inserted with ACCOUNTSUBSCRIPTIONKEY value as 1) but not able to save the value.

org.springframework.dao.InvalidDataAccessResourceUsageException: could not get next sequence value; 
SQL [select ACCOUNTSUBSCRIPTION_PK.nextval from dual];
nested exception is org.hibernate.exception.SQLGrammarException: could not get next sequence value

Table:

CREATE SEQUENCE ACCOUNTSUBSCRIPTION_PK
/
----------------------------------------
-- TABLE: ACCOUNTSUBSCRIPTIONS
----------------------------------------
CREATE TABLE ACCOUNTSUBSCRIPTIONS (
                ACCOUNTSUBSCRIPTIONKEY NUMBER NOT NULL,
                KEY NUMBER NOT NULL,
                EMAIL VARCHAR2(128 CHAR) NOT NULL,
                XEMAIL VARCHAR2(128 CHAR) ,
                DBTIMESTAMP TIMESTAMP (6) DEFAULT systimestamp NOT NULL,
                CONSTRAINT XPKACCOUNTSUBSCRIPTION PRIMARY KEY (ACCOUNTSUBSCRIPTIONKEY) USING INDEX INITRANS 20 TABLESPACE IDX
)
    

AccountSubscriptions.class:

@Entity
@Table(name = "ACCOUNTSUBSCRIPTIONS")
public class AccountSubscriptions implements Serializable{
    private static final long serialVersionUID = 4399781893670770594L;
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "sequence")
    @SequenceGenerator(name="sequence", sequenceName = "ACCOUNTSUBSCRIPTION_PK", allocationSize=1)
    @Column(name = "ACCOUNTSUBSCRIPTIONKEY")
    private Long accountSubscriptionKey;

    @Column(name = "KEY")
    private Long key;

    @Column(name = "EMAIL", length = 128)
    private String email;

    @Column(name = "XEMAIL", length = 128)
    private String xEmail;

    public AccountSubscriptions() {
    }

    public AccountSubscriptions(Long Key, String email, String xEmail) {
        this.Key = Key;
        this.email = email;
        this.xEmail = xEmail;
    }

    public Long getAccountSubscriptionKey() {
        return accountSubscriptionKey;
    }

    public void setAccountSubscriptionKey(Long accountSubscriptionKey) {
        this.accountSubscriptionKey = accountSubscriptionKey;
    }

    public Long getKey() {
        return key;
    }

    public void setKey(Long key) {
        this.key = key;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getXemail() {
        return xEmail;
    }

    public void setXemail(String xEmail) {
        this.xEmail = xEmail;
    }
    @Override
    public int hashCode() {
        int result = this.email != null ? this.email.hashCode() : 0;
        result = 31 * result + (this.xEmail != null ? this.xEmail.hashCode() : 0);
        result = 31 * result + (this.accountKey != null ? this.accountKey.hashCode() : 0);
        return result;


    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        } else if (o != null && this.getClass() == o.getClass()) {
            AccountSubscriptions that;
            label57: {
                that = (AccountSubscriptions)o;
                if (this.email != null) {
                    if (this.email.equals(that.email)) {
                        break label57;
                    }
                } else if (that.email == null) {
                    break label57;
                }

                return false;
            }

            label50: {
                if (this.xEmail != null) {
                    if (this.xEmail.equals(that.xEmail)) {
                        break label50;
                    }
                } else if (that.xEmail == null) {
                    break label50;
                }

                return false;
            }

            if (this.key != null) {
                if (!this.key.equals(that.key)) {
                    return false;
                }
            } else if (that.key != null) {
                return false;
            }

            return true;
        } else {
            return false;
        }



    @Override
    public String toString() {
        return "AccountSubscriptions{" +
                ", key=" + key +
                ", email='" + email + ''' +
                ", xEmail='" + xEmail + ''' +
                '}';
    }
}

Account.hbm.xml

<class name="com.citrix.g2w.services.subscription.domain.AccountSubscriptions" table="ACCOUNTSUBSCRIPTIONS">
    <id name="accountSubscriptionKey" type="long" access="field" >
        <column name="ACCOUNTSUBSCRIPTIONKEY" />
        <generator class="sequence">
            <param name="sequence">ACCOUNTSUBSCRIPTION_PK</param>
        </generator>
    </id>
    <natural-id>
        <property name="key" >
            <column name="KEY" />
        </property>
        <property name="email" >
            <column name="EMAIL"  />
        </property>
        <property name="xEmail" >
            <column name="XEMAIL" />
        </property>
    </natural-id>
</class>

As I am new to hibernate, I have some open Questions:

1. Why it is not getting sequence value? I can see in Oracle DB sequence is created.
2. If we are declare the value in Account.hbm.xml then is it required to use @Entity, @Table , @ID, @Generator, @sequence ,@column etc in class?