Can I buy more storage in my iPhone?

Your phone will always have the same amount of storage it came with, you cannot add more. You can buy space on Apple's synching service, iCloud or you can move data from the phone to your computer or an external drive to create more space.

You cannot expand the storage capacity of your iPhone.

1.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

How to check the storage on your iPhone, iPad and iPod touch

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

2. 

If you wish, you can upgrade the 5 GB free iCloud.

See -> What's the difference between device storage and iCloud storage?

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

See also the following Apple articles:

Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage -> Manage your iCloud storage - Apple Support