Your iPhone or iPad connected to Wifi
Step 1: Go to Settings
Step 2: Open Wifi option
Step 3: Check that Wifi is on, and there is a tick on the connection you made with any device.

It would be recommended if you don’t see any check on your Wifi, it means you didn’t connect to any wifi, so you should try to reconnect with your wifi.

Your iPhone connected to Cellular data
Step 1: Go to Settings
Step 2: Open Cellular data
Step 3: Check that cellular data is on or not

Fix 3: Sign out and Sign in again in Apple ID
Step 1: Open Settings
Step 2: Open [Your Name]
Step 3: Scroll Down and click on Sign out

Step 4: Enter your password and tap Turn off
Step 5: Open Sign In

Step 6: Enter your Apple ID and password
Step 7: Enter six-digit verification code sent on your any trusted device.
Fix 4: App store available in your region
Few times App store face some server problem in some region, so you also need to check that once.
Step 1: Go on the link Apple System Support.
Step 2: Click on All Services
Step 3: Check that App Store showing available.

If it is showing not available, no need to panic wait till it shows available to you.
Fix 5: Clear App Store Cache
As App Store is an application as to why it also possible that it can fail and can have any corrupt bug in that, you can clear that by using this method
Step 1: Open App Store
Step 2: Tap any one of the five tabs 10 times
Step 3: Launch App switcher and close the App Store application
Step 4: Open the App store again and check that the App Store is working or not.
Fix 6: Checking Update of iOS
It is quite possible that you didn’t update your iOS system, and this can lead you to this problem.
Step 1: Open Settings
Step 2: Go to General
Step 3: Click on Software Update
Step 4: Check is any iOS update is pending
If there any iOS update is pending, then just update it.
Fix 7: Troubleshooting software problem
If any of the fixes didn’t work, then this one is the last hope from your side. It can be a deep software failure. So that you need to reset all the settings that can help to fix this issue.
Step 1: Open Settings
Step 2: Go to General
Step 3: Go to Reset
Step 4: Click on Reset all settings

Step 5: Click Reset all settings in the pop-up menu
Fix 8: Hardware problem
If any of the fixes didn’t work out till then, its an issue with your iPhone hardware such as there are small antenna or any of the problems.
So, in that case, you just have to show your iPhone to any of your nearest iPhone care services.