So I do see the error, but what I would like to see, if please go to Planner. Select the Plan. In the URL get the PlanId.
Now, look at the Plan you are using in the Flow to make sure.
Also, you said you added 2 people and a Guest, you didnt say you added yourself. So if you didnt add yourself, and if its your connection to Planner, you won't have access.
To do another test, outside of Planner and outside of Power Automate help. Do this.
This will allow you to do a simple test
You can see here, I am signed in and the its into the Gernaey Tenant. Make sure you are in the same tenant as the Group and Planner
2. Make sure you have the Plan ID from Planner that I asked you to get before.
3. Click on All PLanner tasks for a Plan on the left.
You can see in the URL (right side, next to the dropdown thats says v1.0)
Yo ucan see the {plan-id}
Remove it and add in the PLanId from Planner
4. Click Run Query
See if it gives you an error.
Outcomes
1. If it does NOT give an error
A) You grabbed the wrong guid and you tested with a plan that you have access too
B) You DO have access to this plan, BUT in the flow you are pointing a the wrong one
C) You are simply pointing at the wrong one due to bad flow design