I can't backup my iPhone

Icloud+ upgraded for 2T and my iPhone cannot be completed the backup and my iPhone storage still full.

iPhone storage and having an iCloud+ plan subscription those are totally 2 different things, see -> Related Article: What's the difference between device storage and iCloud storage?

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.

See also the following Apple articles:

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

I can't backup my iPhone

See the following Apple articles:

How to back up your iPhone, iPad or iPod touch with iTunes on your PC

How to back up your iPhone, iPad or iPod touch with iTunes on your PC – Apple Support (UK)

How to back up your iPhone, iPad, and iPod touch with iCloud

How to back up your iPhone or iPad with iCloud - Apple Support

If you can't back up to iCloud

If you can't back up to iCloud - Apple Support