Force quit and relaunch the app. If it still doesn’t work, update the app.
Check your payment method. It is also possible that you have been waitlisted on Instagram.
Try switching to a personal account.
Your payment account is disabled on the App Store or Google Play Store.
You have been waitlisted.
You recently created your Instagram account.
You are using a Professional account on Instagram.
Fix 1: Clear Instagram App Cache
On Android : Long-press the Instagram app icon and select App Info . Then tap on Storage and Cache > Clear Cache .

On iPhone : Open the Settings app > General > select iPhone Storage . Tap on Instagram > Offload App and then tap on Offload again to confirm. Now, reinstall the app to see if the issue persists.

Fix 2: Check Your Payment Account
Fix 3: You Have Been Waitlisted on Instagram
If you have been added to the waitlist for subscriptions, you must hold your horses until the creators make subscriptions available. You will be notified on your account whenever a Subscription from that particular creator is available. Also, your account needs to be at least 30 days old.
Fix 5: Switch to Personal Account on Instagram
Here’s how you can switch to a Personal account.
Step 1: Open the Instagram app on your iPhone or Android phone. Tap your profile icon at the bottom-right corner.
Step 2: Tap the hamburger menu icon at the top-right corner and select Settings and privacy .
Step 3: Select Creator tools and controls and tap on Switch account type .

Step 4: Select Switch to personal account at the bottom.
Fix 6: Check if You Are in an Eligible Country
As per Instagram, you and the creator should be in a country eligible for subscriptions. Subscriptions can be purchased in almost every country except Cuba, Iran, Ukraine, North Korea, and Syria.