[From string (line 7) ]                                                          
                                                                                 
@startuml                                                                        
create B                                                                         
A -> B                                                                           
B -> A                                                                           
...                                                                              
create B                                                                         
^^^^^                                                                            
 You cannot Activate/Deactivate just after a ... (Assumed diagram type: sequence)