Hi @Srecko Suvajac ,
Are you the guest user in Teams channel?
Do you mean you cannot edit the task in Teams Planner?
In my experience, all the members in the Plan will have equal access, I did a test in my environment. Guest and Teams member can edit the assigned task with no issue.
To exclude the client issue, we also recommend you go to Teams web client to see if you can edit the task normally.
If you are convenient, please also provide the screenshot of your issue to narrow down it. Thanks for your understanding!