The plugin has a page that checks your Houzez setup for you, step by step, and tells you what is still missing. Use it.
Where to find it
Go to Mollie Payments, then the Theme activation tab.
It walks you through everything Houzez needs, in order, with no code required.
What it covers
The checklist mirrors what these articles have described.
Your Mollie API keys are entered and the connection works.
Enable Paid Submission is set to something other than No, so a checkout actually exists.
Choose Payment gateway type is set to Houzez Custom Gateways, not WooCommerce.
Your payment methods are switched on at Mollie, and SEPA Direct Debit too if you sell subscriptions.
Your payment methods are switched on in the plugin.
Houzez’s own gateways, Stripe, PayPal and Wire Transfer, are switched off, so your checkout shows one payment section rather than three.
The Stripe fields on your packages are empty, because Mollie does not use them.
Each package has its Payment Type set: one-time or subscription.
Registration is switched on, so new customers can sign up and pay.
Apple Pay and Google Pay, if you offer them.
You have made a test payment.
You have switched to live.
Why bother, if you have read the articles
Because a checklist catches what reading does not.
You will follow every article carefully, and you will still miss one thing, because that is how people work. The checklist takes two minutes and it is the difference between finding your mistake now, or hearing about it from a customer who could not pay.
The other page worth opening
Once you are set up, open the System tab and click Run checks.
It examines your live setup rather than a list of instructions, and it warns you plainly if something is wrong: WooCommerce mode still on, no API key, webhook not configured, a method enabled in the plugin but not at Mollie.
Between them, those two pages answer most of the questions we get asked.