Ubuntu MongoDB起動時、ディレクトリ権限エラーになってしまう

前提・実現したいこと

さくらのVPSのUbuntu 20.04で、MongoDBを起動しようとしています。
ディレクトリ権限に関するエラーが発生したので、権限の設定を行ったのですが、エラーが変わらない状況です。

ご教授いただけると助かります。

発生している問題・エラーメッセージ

MongoDBの起動コマンド（sudo systemctl start mongod）を入力すると、ディレクトリ権限関連のエラーが発生しました。

● mongod.service - MongoDB Database Server
     Loaded: loaded (/lib/systemd/system/mongod.service; disabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Sun 2021-05-16 03:08:26 JST; 1min 12s ago
       Docs: https://docs.mongodb.org/manual
    Process: 143473 ExecStart=/usr/bin/mongod --config /etc/mongod.conf (code=exited, status=100)
   Main PID: 143473 (code=exited, status=100)

var/log/mongodb/mongod.log
エラー全文から抜粋。

2021-05-16T18:20:35.002+0900 I  STORAGE  [initandlisten] exception in initAndListen: IllegalOperation: Attempted to create a lock file on a read-only directory: /var/lib/mongodb, terminating

エラー全文

2021-05-16T18:20:34.975+0900 I  CONTROL  [main] ***** SERVER RESTARTED *****
2021-05-16T18:20:34.976+0900 I  CONTROL  [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 specify --sslDisabledProtocols 'none'
2021-05-16T18:20:35.001+0900 W  ASIO     [main] No TransportLayer configured during NetworkInterface startup
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] MongoDB starting : pid=148310 port=27017 dbpath=/var/lib/mongodb 64-bit host=ik1-408-36284
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] db version v4.2.14
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] git version: 0e6db36e92d82cc81cbd40ffd607eae88dc1f09d
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] OpenSSL version: OpenSSL 1.1.1f  31 Mar 2020
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] allocator: tcmalloc
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] modules: none
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] build environment:
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten]     distmod: ubuntu1804
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten]     distarch: x86_64
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten]     target_arch: x86_64
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] options: { config: "/etc/mongod.conf", net: { bindIp: "0.0.0.0", port: 27017 }, processManagement: { timeZoneInfo: "/usr/share/zoneinfo" }, storage: { dbPath: "/var/lib/mongodb", journal: { enabled: true } }, systemLog: { destination: "file", logAppend: true, path: "/var/log/mongodb/mongod.log" } }
2021-05-16T18:20:35.002+0900 I  STORAGE  [initandlisten] exception in initAndListen: IllegalOperation: Attempted to create a lock file on a read-only directory: /var/lib/mongodb, terminating
2021-05-16T18:20:35.002+0900 I  REPL     [initandlisten] Stepping down the ReplicationCoordinator for shutdown, waitTime: 10000ms
2021-05-16T18:20:35.002+0900 I  SHARDING [initandlisten] Shutting down the WaitForMajorityService
2021-05-16T18:20:35.002+0900 I  NETWORK  [initandlisten] shutdown: going to close listening sockets...
2021-05-16T18:20:35.002+0900 I  NETWORK  [initandlisten] Shutting down the global connection pool
2021-05-16T18:20:35.002+0900 I  STORAGE  [initandlisten] Shutting down the FlowControlTicketholder
2021-05-16T18:20:35.002+0900 I  -        [initandlisten] Stopping further Flow Control ticket acquisitions.
2021-05-16T18:20:35.002+0900 I  INDEX    [initandlisten] Shutting down the IndexBuildsCoordinator
2021-05-16T18:20:35.002+0900 I  NETWORK  [initandlisten] Shutting down the ReplicaSetMonitor
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] Shutting down free monitoring
2021-05-16T18:20:35.002+0900 I  FTDC     [initandlisten] Shutting down full-time data capture
2021-05-16T18:20:35.002+0900 I  STORAGE  [initandlisten] Shutting down the HealthLog
2021-05-16T18:20:35.002+0900 I  -        [initandlisten] Dropping the scope cache for shutdown
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] now exiting
2021-05-16T18:20:35.002+0900 I  CONTROL  [initandlisten] shutting down with code:100

試したこと

権限を付与する2つの方法を試しました。

ディレクトリの権限をchmodコマンドで許可

var/lib/mongodbそれぞれのディレクトリを777に変更しました。

mongodbユーザーにroot権限を付与

visudoコマンドで、以下のようにmongodbユーザーにroot権限を付与しました。

root    ALL=(ALL:ALL) ALL
ubuntu  ALL=(ALL:ALL) ALL
mongodb ALL=(ALL:ALL) ALL

補足情報（FW/ツールのバージョンなど）

MongoDB v4.2.14
Ubuntu 20.04
MongoDB関連のインストール済みパッケージ

mongodb-org-mongos/bionic,now 4.2.14 amd64 [installed,upgradable to: 4.4.6]
mongodb-org-server/bionic,now 4.2.14 amd64 [installed,upgradable to: 4.4.6]
mongodb-org-shell/bionic,now 4.2.14 amd64 [installed,upgradable to: 4.4.6]
mongodb-org-tools/bionic,now 4.2.14 amd64 [installed,upgradable to: 4.4.6]
mongodb-org/focal,now 4.4.6 amd64 [installed]