Cannot update mailbox /var/mail/root

Dear All

I can’t send email out. In snappy mail it says “Can’t send email”. I look into /var/log/mail.log. Here is the last 10 lines. Can you please tell me what’s wrong?

Sep 14 21:14:14 localhost dovecot: imap([email protected])<14633><ppfAglsF2OZ/AAAB>: Disconnected: Logged out in=11 out=491 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0
Sep 14 21:14:14 localhost postfix/local[14644]: C1C7F306488: to=<[email protected]>, orig_to=<postmaster>, relay=local, delay=0.2, delays=0.11/0.03/0/0.06, dsn=5.2.0, status=bounced (cannot update mailbox /var/mail/root for user root. unable to create lock file /var/mail/root.lock: No such file or directory)
Sep 14 21:14:14 localhost postfix/cleanup[14637]: EFC7630AE0A: message-id=<20230915011414.EFC7630AE0A@localhost>
Sep 14 21:14:15 localhost postfix/bounce[14645]: C1C7F306488: sender non-delivery notification: EFC7630AE0A
Sep 14 21:14:15 localhost postfix/qmgr[13071]: EFC7630AE0A: from=<>, size=2895, nrcpt=1 (queue active)
Sep 14 21:14:15 localhost postfix/qmgr[13071]: EFC7630AE0A: to=<[email protected]>, relay=none, delay=0.06, delays=0.06/0/0/0, dsn=2.0.0, status=sent (undeliverable postmaster notification discarded)
Sep 14 21:14:15 localhost postfix/qmgr[13071]: EFC7630AE0A: removed
Sep 14 21:14:15 localhost postfix/qmgr[13071]: C1C7F306488: removed
Sep 14 21:14:15 localhost dovecot: imap-login: Login: user=<[email protected]>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=14647, TLS, session=<mMTOglsF4uZ/AAAB>
Sep 14 21:14:15 localhost dovecot: imap([email protected])<14647><mMTOglsF4uZ/AAAB>: Disconnected: Logged out in=91 out=558 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0