Hi Abdullah_k_y.
Welcome to the Apple Community. We are all Apple users, like yourself (not Apple itself).
There may be any one of a few different reasons you can not sign in
Apple support have articles online (links below) that may help troubleshoot.
✽. There may be a Screen Time setting that restricts purchases :(ie: don't allow / allow)
See this article : Set up Screen Time for yourself on iPhone – Apple Support
(ie: Check: Settings > ScreenTime > Content and Privacy Restrictions > iTunes & App Store Purchases)
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✽ Your watch / iPhone may need to be logged in with your Apple ID
before you go to the App Store to download free apps or to make purchases
How to: Sign in with your Apple ID
Sign in with your Apple ID - Apple Support
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✽. If this is the first time you are using the App Store or iTunes:
You may need a payment method listed in your Apple ID account for this first time.
Change, add, or remove Apple ID payment methods - Apple Support
Trouble-shooting: f needed:
If you can't open the App Store or iTunes Store, buy content, or edit payment information - Apple Support
Or
If you still need help:
The App Store Support Team has a website online.
(There is a "Get Support" link to contact them, at the very bottom of their webpage.)
App Store - Official Apple Support
All the best :-)