Open the device's "Settings" app, select this app ("Simple Diary") from the app list, tap "Photos", and allow full access to your photo library. If the camera does not work, go back to this app's settings page and turn on "Camera".
If you cannot find the setting, or it does not fix the problem, please reinstall the app using the method below, which keeps your data safe while reinstalling.
Open the Settings app, tap "General", then "iPhone Storage", select this app, and tap "Offload App".
This removes the app itself while keeping its data on your device.
Then search for this app on the App Store, install it again, and check whether you can attach photos.
Thank you.