Help! Couldn't add New IPsec Peer - Unsafe configuration.

I’m ready to config IPsec with aggressive mode. But it’s appear error.

[admin@MikroTik] > /ip ipsec peer add address=1.1.1.1/32:500 auth-method=pr
e-shared-key secret=“xxx” exchange-mode=aggressive send-initial-contact=y
es nat-traversal=yes proposal-check=obey hash-algorithm=md5 enc-algorithm=des dh
-group=modp768 generate-policy=yes lifetime=28800 dpd-interval=disable-dpd
failure: Unsafe configuration. Suggestion to use certificates

Please help me, thanks!

RouterOS v3.20