Hi @ ViniciusAE,
Could you please share a screenshot of the configuration of your flow?
For your first problem:
Do you want to get the description of the task in Planner?
You could add a "Get task details" action on your flow, and then you could choose the "Description" dynamic content of the "Get task details"action,please refer to screenshot below:

For your second problem:
I have made a test on my side, and the issue that you mentioned is confirmed on my side.
The "Created By Display Name" dynamic content of the "Create a task" action would be null when the flow run, I would post this issue to my product team, if the issue is solved, I will reply here.
As an alternative way,you could add a "Get user profile" action on your flow, and you could fill in the input field of the "Get user profile" action with the "Created By User Id",and then you could get the user's display name by choosing the "Display Name" dynamic content of the "Get user profile" action, you could refer to screenshot below to create the flow:

Regards,
Alice Zhang