Hi, MWCCTHEO
Thanks for reaching out.
My name is God'swill, I'll be happy to help you today.
Based on the information you provided, you have added members in the plan, but when you create a new task and try to assign it to people, the members list doesn't display and you cannot select people from the list.
(If I misunderstand anything, feel free to correct me)
For troubleshooting, you may try to use the private window of the browser, or use other different browsers to check the result.
If issue persists, in this situation, I recommend you inform your Microsoft 365 admin of this issue, let him help you create a support ticket via Microsoft 365 Admin Center>Support>New service request. The support team there has the correct escalation channel of this kind of issue, this is also the most efficient way in handling the case for you.
Please be sure to let me know if you need more help as I'll be glad to do so.
Regards...
God'sWill