You have to open the 3uTools of your iDevice when you import photos from PC to your iPhone’s Camera Roll. And then you have to allow the 3uTools to access your iPhone’s Photos. Then after can you import the photos from PC to your iDevice.
If you open the 3uTools on your iDevice for the first time, the 3uTools will send you a reminder that “Mood diary” would like to Access Your Photos”. You just need to click on the button “OK”, and then the 3uTools will import photos from PC to your iDevice.
If you carelessly click on the button “Don’t Allow” which means you refused the request that the 3uTools wants to access your photos, then please return to the settings of your iDevice and reset it.
Please Open the “Settings” of your iPhone →click on the button “Privacy”→click the button “Photos”→find the 3uTools and then turn on the button behind the 3uTools.