[180] load_tpm_key: Failed to load.

hello, upon getting that error message after upgrading to 7.4.8 through the console, is your Fortigate able to boot successfully or just gets stuck at that error message? That error message is certainly not normal and I'd suggest you to open a support ticket with TAC. Did you follow the upgrade path when upgrading the Fortigate?

 I followed the FortiGate upgrade path to upgrade, and FortiGate able to boot successfully, but the message "[180] load_tpm_key: Failed to load." was displayed on the console.

Upgrade Path：7.28 → 7.2.11 → 7.4.8

I'm seeing same thing. Guessing it has to do with this:

https://docs.fortinet.com/document/fortigate/7.4.0/new-features/435758/fortigate-identity-stored-in-tpm-7-4-8