Hi @Adam and welcome to the forum.
Your passphrase has to be at least 8 characters long, otherwise Checkmk won’t take it.
I went through the SNMPv3 RFC (https://www.rfc-editor.org/rfc/rfc3414) but couldn’t find the 8 characters as a requirement (the min. length is based on the used security protocol).
I also asked ChatGPT about it and this was the response: 
In SNMPv3 specifications, the minimum letter count (also known as the minimum password length) for the authentication and privacy (encryption) keys is determined by the security level and the chosen security mechanisms.
In SNMPv3, there are three security levels defined:
No Authentication and No Privacy (noAuthNoPriv): In this security level, there is no requirement for a password or key, so the minimum letter count is not applicable.
Authentication but No Privacy (authNoPriv): In this security level, only authentication is applied, and no encryption is used. The minimum letter count for the authentication key (password) is usually 8 characters.
Authentication and Privacy (authPriv): This is the highest security level where both authentication and encryption are used. The minimum letter count for the authentication key is usually 8 characters, and for the privacy key (encryption key), it is often 8 to 16 characters.
It’s essential to note that these minimum letter counts are not strict requirements imposed by the SNMPv3 standard itself but rather practical recommendations to ensure adequate security. Actual implementations might have varying requirements and guidelines.
Additionally, SNMPv3 supports various authentication protocols like MD5 and SHA-1, and encryption protocols like DES, AES, etc. The specific choices of these protocols can also influence the required key lengths for authentication and privacy.
So please make sure your used password is longer than 8 and not longer than 16 characters and you should be good to go.
Regards
Norm
