I have an older iPhone, so I know exactly what that means, but do you know how much data storage your iPhone SE has? If it's 16GB, your device has very limited storage space. It's very possible the message from Apple means they want you to purchase (upgrade to) a new device, with more space to download that specific app. Can you tell us which app you want to use from the App store? You can also contact the app developer for help. Here's a link to help you do just that: https://support.apple.com/en-us/HT207959:
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