Fix 1: Use iTunes to delete photos on iPhone/iPad
Under the first circumstance, you are suggested to apply iTunes to delete photos from your iPhone/iPad. Related operating steps are shown below.

Step 1: Connect your iPhone/iPad to computer and launch the iTunes app.

Step 2: Tap on your iPhone/iPad icon and go to the Summary option.

Step 3: Click Photos tab and uncheck the photos you want to delete on your device.

Step 4: Finally hit on the Apply button to sync iTunes with your iPhone.

Fix 2: Disable iCloud Photo Library button
If you are in the second situation, then you could follow the steps here to fix can’t delete photos from iPhone/iPad.

Step 1: Head to Settings > Photos & Camera on your iPhone/iPad.

Step 2: If you turned on iCloud Photo Library, close Optimize iPhone Storage option.

Step 3: Or you could directly switch iCloud Photo Library button off.

Fix 3: Have PC Expert Services Repair your iPhone/iPad
However, if the problem of photos not deleting from iPhone/iPad results from iOS bugs, then you might have to use professional repair shop, for instance, PC Expert Services to fix it completely. PC Expert Services not only can help you get out of trouble without data loss, but also improve your iOS device’s performance through removing other potential dangers.