Hi @Michael Wang  Welcome to Box Community!

When assigning Tasks in the Workflow editor, external collaborators will not appear in the Task's "Assignee(s)" list unless the user editing the Workflow is also the Task item's sole owner. You may want to try these workarounds below:

• The workflow creator can remove and re-invite the external collaborator to the folder before they're able to assign that person a task in the workflow.
• The workflow creator can invite the external user as a collaborator on any content that they own and then the external user should appear in the Relay task search results

Also as described from Box Relay FAQ > What do I do if an external user is not appearing for task assignment and/or as a mail recipient?

If you're the item's owner, create a workflow as item owner, and assign the external user.
If you're not the item's owner, create a workflow and invite the external user with the following conditions:

• The external user's account must exist before being invited.
• The external user must choose to automatically accept collaboration invitations.
• The invitation must not be pending with factors described here.

Looking forward to sort things out. 

Hi Jey,

Following are my feedback with blue words.

• The workflow creator can remove and re-invite the external collaborator to the folder before they're able to assign that person a task in the workflow. Michael: yes, removed and re-invited external collaborator again, but I cannot assign a taks for external collaborator in the workflow 
• The workflow creator can invite the external user as a collaborator on any content that they own and then the external user should appear in the Relay task search results Michael: we invited the external user and the external user can access our box folder. But the external user did not appear in the Relay task search results.

• If you're not the item's owner, create a workflow and invite the external user with the following conditions:

1. The external user's account must exist before being invited. Michael: yes, external user's account are exist before invited.

2. The external user must choose to automatically accept collaboration invitations. Michael: yes, external user must choose to automatically accept collaboration invitations
3. The invitation must not be pending with factors described here. Michael: yes, is not pending.

Thank you.

Hello @Michael Wang 

I appreciate your time checking on this and thanks for bringing this up to our attention.

As these workarounds still did not help appearing the external users for you to add to the Workflow, this would require further troubleshooting steps with the help of our Product Support. To assist you, kindly open a support ticket. You may also provide the screenshot if you encountered an error.

Once a case is submitted, one of the specialists from our Support Team will reach out to check your settings and continue assisting you. Hope to have this resolved soon! 

clicked your link and tried to open a ticket. But got below message.

Box Support -> Submit a request

Need Help?

Your organization has requested that you contact your internal Help/Service Desk.
Please contact them directly for further assistance.
Click here to view tickets on which you are already copied.

Hello @Michael Wang 

Sorry about that! Regarding this message, it looks like your organization has a unique ticketing system where users require to submit in your internal tool. 

With that, a ticket cannot be submitted directly to Box Support since you have a Helpdesk model. To help resolving the concern about Relay workflow, first, you may get in touch with your Helpdesk. Your admin will then submit a ticket to Box Support if needs additional assistance from the Box side.
 

We appreciate your continued patience with this matter and I look forward to have this taken care.