cannot register

got an error during registration:

com.atlassian.confluence.core.InfrastructureException: You are not licensed to add any more users to this installation of Confluence.
at com.atlassian.confluence.user.DefaultUserAccessor.addUser(DefaultUserAccessor.java:49)
at sun.reflect.GeneratedMethodAccessor784.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:59)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:201)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:176)
at com.atlassian.confluence.util.profiling.SpringProfilingInterceptor.invoke(SpringProfilingInterceptor.java:17)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:196)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:135)
at $Proxy54.addUser(Unknown Source)
at sun.reflect.GeneratedMethodAccessor784.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:59)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:201)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:176)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:153)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.