To use the sever edition you need to pay for it. https://server.manager.io/buy
There is a free trial only while you evaluate if you want to use it. As soon as you start to use it for your accounting, the free trial is over.
NG software uses an honesty system to enforce this which is viable if it is not abused. For the benefit of all users do not abuse the trust you are given.
For a similar discussion see Is Manager.io (server edition) is free for lifetime? - #4 by Patch
To use the purchased edition together with 1 yr of updates. After that there is a Maintenance fee for another 12 months of updates.