On SFMC / Setup / Settings / Security / Security Settings / Restrict Logins by IP Address (IP Whitelisting): if your account is set to "Log Violations and Deny Access", this may prevent third-party services such as Playable from connecting to your SFMC account to add your video Embed codes.
To fix this, you need to whitelist the IP addresses used by Playable servers. Click on "Login IP Allowlist" and try the following IP ranges:
126.96.36.199 - 188.8.131.52
184.108.40.206 - 220.127.116.11
For a more detailed explanation and the most recent instructions, please see here: