thanks a lot for the comments.

I tried all these steps. But when I click firewall2 in YAST, it asked me to install it. Then it failed with messages:

Subprocess failed. Error: RPM failed: warning: /var/cache/zypp/packages/SUSE_Linux_Enterprise_Desktop_12_SP3_x86_64:SLED12-SP3-Updates/noarch/SuSEfirewall2-3.6.312.333-3.13.1.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 39db7c82: NOKEY
error: db4 error(-30986) from dbcursor->c_get: DB_PAGE_NOTFOUND: Requested page not found
error: error(-30986) getting "/sbin/" records from Dirnames index: DB_PAGE_NOTFOUND: Requested page not found
error: db4 error(-30986) from dbcursor->c_get: DB_PAGE_NOTFOUND: Requested page not found
error: error(-30986) getting "/sbin/" records from Dirnames index: DB_PAGE_NOTFOUND: Requested page not found
error: SuSEfirewall2-3.6.312.333-3.13.1.noarch: install failed