Hi @Mahin Ahmed,
Did you activate your Office? If not, please sign in your Microsoft 365 account to activate Office products. Then, see if the Designer appears.
I find the requirement for Designer feature in this article.
From your description, apparently yours fits it. So, if the issue persists after signing in the Office products, I suggest you contact Microsoft support to reach out to Microsoft support team for further assistance. Experts there will help you check from the backend.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.