My camera app closes when I try to take a picture

Nashl ~ Is the battery in your iPhone 7 the original?...

If the battery is weak from old age "performance management" effects come in to play such as:

• Longer app launch times
• Lower frame rates while scrolling
• Backlight dimming (which can be overridden in Control Center)
• Lower speaker volume by up to -3dB
• Gradual frame rate reductions in some apps
• During the most extreme cases, the camera flash will be disabled as visible in the camera UI
• Apps refreshing in background may require reloading upon launch

[Source: iPhone Battery and Performance]

You can find Apple Support Articles such as the above by searching Apple's Knowledge Base.

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First, see how much storage you show at Settings > General > About > Available. You are looking for a minimum of 2GB here.

Then follow https://support.apple.com/en-us/HT201398