Close Project When Tasks Are Complete

I have a projects table and a tasks table. Tasks are a reference under projects as “related tasks”. I would like to set up an action that would mark a project as "complete’ when all of the related tasks are all marked “complete”.

You can set an action in Projects table like this:
Set an action with Data: set the value of a column, select your column i.e. [Status] and set the –To this value property to Complete
Under BEHAVIOR section, locate the Only if this condition is true property and set below expression:

COUNT(SELECT([Related Tasks][KeyColumn],NOT([Status]="Complete")))=0

The action will be shown in the Detail View of any particular project, if and when this expression evals to TRUE.