Hello @Farrukh Riaz ,
The "Previously Assigned" status in the training completion context means the user was assigned the training in the past and hasn't completed the training yet. The user can complete the training module, at which the Status value changes to Completed.
Can I know the user has completed the training completion and still showing as Previously assigned? If yes, then to remove the training threshold and always assign training, regardless of whether a user has already completed or been assigned a training, set value to 0.
For additional information you can follow: https://learn.microsoft.com/en-us/defender-office-365/attack-simulation-training-settings?view=o365-worldwide#configure-the-training-threshold
Hope this information helps. Let us know if you have any additional queries. Happy to assist you further.