I want to buy more space online for my iPhone but it’s not working please help me

If you are out of space in iCloud, you can certainly increase the space on iCloud, but that is for backups only. If you are low in storage on your iPhone itself, there is no way to add memory to it. Buying iCloud storage will not add space to your phone. So, if you are low in memory, the only way to get space back is to delete things off your phone you don't need or use, like Apps, Music, Photos, Messages, Email, Files, etc.

Read this link about the difference between iCloud and device memory --> What’s the difference between device storage and iCloud storage? – Apple Support (UK)

Next time you purchase an iPhone, make sure you buy one with more memory than you think you need. Once purchased, it can never be increased.

This is not a chat service. You can’t add any more storage to your phone. You can upgrade your iCloud storage if needed.