I attempted to run my OTREE experiment over MTURK today and there was a bug in the code that affected participants payoffs. I shut it down but I have no way of contacting MTURK workers. Since the code was programmed in an old version of OTREE I couldn't use OTREE HR and now I see no option for contacting MTURK workers. The hit only appears on the oTREE side and there is nothing to track on the MTURK site. Is there some other way to contact workers about the study?

There is, using the AWS MTurk CLI: https://awscli.amazonaws.com/v2/documentation/api/latest/reference/mturk/notify-workers.html

You need to configure it correctly and need the Worker-IDs of the participants, then you should be able to contact them.

There is one caveat: It only works if you have approved or rejected their assignments.

Thanks for the reply. So I looked into the CLI earlier and I only receive error messages when I try to run the suggested code. I played with the configuration to get it to work and after searching on forums I cannot figure it out. 

I run this code and receive this error. I tried it with multiple worker IDS who have worked for me previously. 

aws mturk notify-workers --subject "" --message-text "" --worker-ids "”

I always get this error message back.
{
    "NotifyWorkersFailureStatuses": []
}

I also tried doing it using aws-shell but when I try to configure that code I receive a different error message.  'Namespace' object has no attribute 'cli_binary_format'

I think that is the message you get when everything works and the message was sent to the worker :)

Oh Thanks! That is very confusing.