At the top of the plugin there is a badge. It is the fastest way to know whether you are actually able to take money, and it never lies.
Learn to glance at it. It will save you a lot of digging.
The states
TEST
Test mode. No real money. This is what you want while you are setting up, and it is exactly what you do not want after you launch.
LIVE
Real payments are being processed. Everything is connected and working.
LIVE, NO KEY
This is the one to watch for. Live mode is selected, but you never pasted in a live key.
Nothing can be paid. Your checkout will simply fail, and nothing on your website will explain why to your customer. They will assume you are broken and leave.
Go to the API keys tab, paste in your live key, and save.
No API key
Nothing is connected yet. Normal when you have just installed the plugin, a problem at any other time.
Connection issue
The plugin tried to reach Mollie and could not.
This is usually one of three things. Your key is wrong. Your host is blocking outgoing connections. Or Mollie itself is having a bad day, which does happen occasionally and is not your fault.
Click the badge to run a fresh check. If it keeps failing, open the API keys tab and click Test connection for a clearer answer.
Make a habit of it
Before you announce that you are open for business, look at the badge.
After any change to your keys or your mode, look at the badge.
If a customer says they could not pay, look at the badge first, before you look at anything else.