Can I buy iPhone storage

No you will need to remove some data from your device to free up space. Otherwise you will need to upgrade to an iPhone with more physical storage. If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

In addition to KaeBFly’s response:

JaydaBernard16 wrote:

I’ve used all my storage

Be certain you never have less than 6GB of iPhone storage free. (Settings > General > iPhone Storage)