If the completion of the maintenance plan is strongly tied to the present time and the specific moment of completion is not mentioned and not important then the first form is perfect:
• I have completed the maintenance plan.
If the completion is tied to a specific moment in the past such as last year then you should use the past simple:
• I completed the maintenance plan last year.
Only if the completion of the maintenance plan preceeded another past would you need to resort to the past perfect:
• I had completed the maintenance plan and was working on a new project at the time.
I hope that helps.
Student or Learner