First, visit the G Suite Marketplace Blissfully App, and choose Domain Install to begin installation. From there, you will be redirected back to Blissfully, and guided through next steps.
After the app has detected you've installed the G Suite Marketplace app, it needs to confirm two other facts:
That you are a G Suite domain administrator (Super Admin).
That API access to G Suite is enabled.
If the app instructs you to enable API access to G Suite, visit your G Suite Security Settings. (Tip: If you are asked to log in, re-click this link to take you precisely where you need to be!) You should see an Enable API checkbox. Check it and select Save then return to the Blissfully App - the installation screen should confirm you're good to go.