Finally, I find the problem:
POST https://mysite.com/api/posts
{
"errors": [
{
"code": 500,
"title": "Internal server error",
"detail": "exception 'TijsVerkoyen\Akismet\Exception' with message 'Invalid API-key' in /var/www/mysite/vendor/tijsverkoyen/akismet/Akismet.php:93\nStack trace: