Cause 

The work plan will be locked while it is being edited. If another person tries to edit the same plan at the same time, it be view only. This will help to ensure multiple people do not create scheduling conflicts by trying to edit the same plan at the same time.

Solution

Be sure to check with the user listed in the warning to ensure that they have completed and saved their updated. Then you can refresh your page.

If the warning is still on screen after refresh, click Unlock. 

NOTE: We see this happen sometimes when we are working with schedules, also - it says it's locked by me. If you navigate away from the page, or the page remains idle too long, it will sometimes lock. The good thing is in that case, you can just click "unlock". The lock notification prevents the loss of data and prevents other users step over to the work you are working on.

Example:

Both my accout(Xiaowen Chen) and Support account(Opservice) got shared with the same project "Work Plan Lock Test".
 
Situation 1:
 

1. OPservice get into the WorkPlan and stay, with no editing action
2. Xiaowen get into the WorkPlan at the same time, lock notice shown
3. OPservice did not make any editing and navigate away from work plan
4. Xiaowen refresh the page, nothing edit in work plan, and the lock notice is gone

 
Situation 2:

1. OPservice get into the WorkPlan and stay, with editing action
2. At the same time, when Xiaowen goes into the WorkPlan
3. OPservice save the editing
4. Xiaowen can do 2 actions (both works)

NOTE:

1. You can check on audit for editions
2. If you refresh your page and still see the lock notice, this mean the other user still on work plan page and did not navigate away yet.
3. Recommend to train users to don't work on the same Work Plan schedule at once in OnePlan.
4. If there is any big changes for the work plan, users can easily save a version to prevent any accident overwrite.