Failure : Alice Diop can't be assigned the B2B Buyer Permission Set One Seat permission set license, because Alice Diop's user license doesn't support it.
I don't think you can assign B2B Buyer Permission Set to these users.
Buyer users require a Customer Community license or a license with a higher level of privileges. Buyer managers and account switchers require a Customer Community Plus license or a license with a higher level of privileges.
Permission Sets for Buyers, Buyer Managers, and Account Switchers for B2B Stores
https://help.salesforce.com/s/articleView?id=sf.commerce_buyer_and_buyer_mgr_perm_sets.htm&type=5
Control what Your Users have Access ライセンスの違いでアサインできない
I've been able to assign the permission set group to Anna now. I deleted the permission set and created a new one and then the assignment was successful.
これで、権限セット グループを Anna に割り当てることができました。権限セットを削除して新しい権限セットを作成したところ、割り当ては成功しました。