how i make a video (longer than a few seconds) on my iPhone 7?

If you are unable to record videos longer than a few seconds, you may be full or approaching full on device storage. If your device storage is completely full or almost completely full, you may not be able to take any more photos, record videos, or download any more apps until the some files/apps/photos/videos/etc are removed. This is because there is no physical space left on the storage chip. Imagine you have a 2"x2" box, if your fill it up to the brim with water, you will not be able to add any more to the box without removing some first.

To see how much storage you have

1. Go to Settings.
2. Tap General.
3. Tap [device] Storage. 

You might see a list of recommendations for optimizing your device storage, followed by a list of installed apps and the amount of storage each one uses. Tap an app's name for more information about its storage.

If you're using iOS 10 or earlier, below Storage, tap Manage Storage to see which apps take up the most space.

In order to free up device storage, you will have to delete applications, files, photos, etc. You can move the data to iCloud and delete it from your phone. You can make storage space available on your iPhone, iPad, or iPod touch when you use iCloud Photos and turn on Optimize Storage. Or you can remove content from your apps—like photos, music, and Podcasts—or delete the apps that you're not using.

One alternative storage location for your photos/videos/files is iCloud

When you set up iCloud, you automatically get 5GB of free storage. You can use that storage space to back up your device and to keep all of your photos, videos, documents, and text messages securely stored and updated everywhere. Depending on the size of your backup and the amount of content you keep in iCloud, you might run out of free space. You can make room when you buy more iCloud storage or when you delete content.

I hope this helps you with your issue

https://support.apple.com/en-us/HT207260

To record a video on your iPhone...

1. launch the camera application
2. slide the slider on the bottom bar to the left until you reach the VIDEO option
3. click the red button to begin recording