I tried to pay for more storage but their is a problem

Whatever the problem is, if you are looking to buy more storage, you cannot EVER increase the amount of storage on your iPhone. Buying more storage ONLY increases the amount of space you have to sync data with iCloud. If you are out of space on your iPhone, buying more iCloud Storage, will NOT help you. The only way to get space on an iPhone if you are running low is to delete things you don't need, like, Apps, Photos, Music, Files, Messages, Notes, Email, etc.

Read this --> What's the difference between device storage and iCloud storage? - Apple Support

Ps: If you need more space on your iPhone, you didn't purchase an iPhone with the right level of memory you need. The next time you buy an iPhone, buy one with the most memory you can afford. Once purchased, the memory on iPhone is fixed forever.

What is the problem? We need more information in order to help.

To help other Apple users answer your question:

• Offer as many details as possible about the problem. What were you doing or tryin to do ? What happened? 
• Did your device show an error message? Did it include instructions? If so, what was the message and did you follow their directions? 
• List any trouble shooting steps you have already tried. 

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Please read the following article then post back with your question –

Writing an effective Apple Support Commun… - Apple Community