The full wording is usually "Cellular Plans From This Carrier Cannot Be Added" or something close to it. It appears while installing, before the plan is on your phone.
The usual causes, in order
Your phone is carrier locked
The most common cause by far. A locked phone refuses profiles from other operators. Ask your carrier to unlock it. See "Is my phone carrier locked?" on the SimFuse site.
The code has already been used
If you have installed this plan before, or scanned the code on another device, it cannot be added again. See "This code is no longer valid".
The phone already holds as many profiles as it can
Delete an old eSIM you no longer need and try again. Be careful which one you delete. See "Deleting an eSIM, and when not to".
A poor or filtered connection
Installing needs a working internet connection, and it needs to reach the provisioning servers. Try again on a different Wi-Fi network, off any VPN, and off a corporate or hotel network if you can.
Software out of date
Update your phone, restart it, and try again.
If none of that helps
Contact us with your ICCID, your exact phone model, its software version, and a photo or screenshot of the error. Some models refuse profiles for reasons specific to the market they were sold in, and knowing the model is what tells us whether that is what you are hitting.