Run Queueable as sequence

Question

Good day Everyone,

I have a trigger that will call a Queueable. The process looks like this :

Trigger :
for(User userData : Trigger.new) {
if(Limits.getLimitQueueableJobs() - Limits.getQueueableJobs() > 0 && !Test.isRunningTest()) { 
    system.enqueueJob(new Assignment(userData)); 
} else {
    System.debug('FUTURE LIMIT EXCEEDED');
}

}

and my Queueable looks like this :

public class Assignment implements Queueable, Database.AllowsCallouts {
    // the constructor is parameter is used in execute method
    public Assignment(User theUser) {   
    }

    public void execute(QueueableContext context) {
        // some process here
    }
}

At the moment everything is working fine but what i'm trying to achieve is that the next Queueable will only run if the first Queueable is done running. What I am trying to achieve is something like a chaining Queueable. I did try to update my trigger to only call the Queueable once then I update my Queueable class to something like this :

    Trigger : 
system.enqueueJob(new Assignment(Trigger.new)); 


Queueable:
public class Assignment implements Queueable, Database.AllowsCallouts {
    public List<User> allUserList = new List<User>();

    // the constructor is parameter is used in execute method
    public Assignment(List<User> userList) {   
        allUserList = userList;
    }

    public void execute(QueueableContext context) {
        // some process here



        // at the end of the execute method
        allUserList.remove(0);
        if(allUserList.size() > 0) {
            system.enqueueJob(new Assignment(allUserList)); 
        }
    }
}

But when I update many data simultaneously, I encounter this error.

Too many queueable jobs added to the queue

Can you please give your idea what do I need, to do this ? Thank you.