Can we delete files attached in contact related list, if a field is checked to true i want my file to get deleted in the related list, Is it possible?

Apex trigger challenge (Bulk Apex triggers):
Its working but trailhead is not accepting ,any clues what is this error about.I have checked all the flows ,approvals nothing is related to oppportunity but still it is throwing me an error.

Challenge not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_EXECUTE_FLOW_TRIGGER, The record couldn’t be saved because it failed to trigger a flow. 
A flow trigger failed to execute the flow with version ID 301280000008h7f. 
Contact your administrator for help.: []

Iam getting mail as follows:


This report lists the elements that the flow interview executed. The report is a beta feature.
We welcome your feedback on IdeaExchange
Flow Details

Flow Name: Opportunity

Type: Workflow

Version: 1

Status: ACTIVE

i have a object called stock in that i have a field quantity and other object i have is order in that object i have a field quantity ordered so if quantity order is more than quantity it should throw an error.The two objects are not related by master detail so by which way the task can be done 

I have a object called partname which is a lookup on other object called stock ,how to display partname records using visual force?

Can we delete files attached in contact related list, if a field is checked to true i want my file to get deleted in the related list, Is it possible?

Apex trigger challenge (Bulk Apex triggers):
Its working but trailhead is not accepting ,any clues what is this error about.I have checked all the flows ,approvals nothing is related to oppportunity but still it is throwing me an error.

Challenge not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_EXECUTE_FLOW_TRIGGER, The record couldn’t be saved because it failed to trigger a flow. 
A flow trigger failed to execute the flow with version ID 301280000008h7f. 
Contact your administrator for help.: []

Iam getting mail as follows:


This report lists the elements that the flow interview executed. The report is a beta feature.
We welcome your feedback on IdeaExchange
Flow Details

Flow Name: Opportunity

Type: Workflow

Version: 1

Status: ACTIVE

i have a object called stock in that i have a field quantity and other object i have is order in that object i have a field quantity ordered so if quantity order is more than quantity it should throw an error.The two objects are not related by master detail so by which way the task can be done 

I have a object called partname which is a lookup on other object called stock ,how to display partname records using visual force?