When creating a client part of the creation process must involve creating a "Client Contract" for the client record. The client contract is what is going to activate specific products for that client, and the associated client users.
You can add a client contract by editing the client record, and clicking the "Client Contract" button:
From here you will see if you have a contract or not. If you don't have a contract you will need to add a contract by clicking "Add":
Now, this is how you'll track how you've licensed your client. If it's an internal department it may be easiest to just create a 99 year contract or something to indicate it's a permanent contract with you. However, if you brought in a client of your own (say you were consulting or something) you could limit their access to the tool by adding specific information on the contract length, etc. The major items to complete here are going to be the start/end dates based on the contract length as that helps define what products are active and for how long. Once you've completed this step, click "Save":
When you click "Save" it will bring up the remainder of the contract where you will add the specific line items that will activate specific product modules, and/or define limits on how many users or vendors may be added to this client. (This is the cause of your initially reported error.)
By clicking the "Add" button