Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
To ensure I fully understand your situation and provide the most effective assistance, could you please help confirm the following:
What licenses are assigned to the affected user and to a working user who can see the Trial Active button? Please compare the assigned licenses and service plans for both accounts.
Has the affected user ever activated a Planner Premium, Project, or any related trial previously?
As far as I know, there is no dedicated setting in Teams or Planner that can be manually enabled to make the "Trial Active" or "Timeline" button appear for a specific user.
The Timeline view is a Planner Premium feature and is available only to users who have the required Planner/Project entitlement or are eligible for the Planner Premium trial. Likewise, the Trial Active (or trial offer) option is displayed only when the user qualifies for that trial experience.
Since users within the same tenant can have different license assignments and trial eligibility statuses, I would recommend comparing the assigned licenses and service plans between the affected user and a user who can see the trial option. Additionally, please confirm whether the affected user has previously activated a Planner Premium or Project trial, as this may affect the availability of the trial offer.
You can refer more via: How to access a 30-day trial of Planner's premium features
I hope this helps.