Contact is not recored in sendinblue

I have tried to integrate this plugin. But it seems, it does not work.

1) I put the correct V2 Sendinblue Api at the end of the wp-config-file.
2) i created an contact-form with the parameters of the Readme-File.
3) I put the correct contact-list-number inside the contact-form

But nothing is shown at the contact-list at the sendinblue website.

(I am not using the official sendinblue-plugin. Is it needed? Using Divi Bloom-Plugin works perfect. But want to have a slim and lightweight version.

I would be very thankful for support / help.
You can see the steps, taken here:

https://gofile.io/?c=uaD38O