This Error tells you, that you are using the LDAP Proxy and have more than 50 users with active tokens in your system, which would require a subscription for the LDAP proxy.
Open Source does not mean, that everything works out of the box. Or that everything works. Or that everything is free of charge. Open Source and Costs have nothing to do with each other.
Software is never Free Of Charge. It requires the time of the developers, of you or of your IT employees.
Some peolple claim, that Open Source is even more expensive than proprietary software. I would not say this, but I personally am very happy to pay for certain Open Source software. I am using Open Source due to the vast possibilities and flexibilities. Not only due to costs. Think about why you are using Open Source! Think about why you are using privacyIDEA! Have you read this?
privacyIDEA Server is Open Source. It does not require any subscription.
The privacyIDEA LDAP Proxy is Open Source. But for more than 50 users, you need a subscription.
If you know someone who can create a subscription, you can put whatever meaning you want to into this subscription. (The company I work with puts the meaning of “hey we provide support for you” into this subscription).