If you are using a Mac computer and an Android phone, Google provides a useful tool called Android File Transfer to transfer photos, videos and files between the devices.
Could not connect to device
Try reconnecting or restarting your device
I tried some solutions suggested by Google like trying different USB cables, rechecking MTP, restarting devices but they didn’t work. After some research, I found out that the issue came from Kies drivers that I might have installed in the past. Here is how to remove those drivers and make Android File Transfer work again.
1. Visit Kies website to download Kies for Mac
2. Open the KiesMacSetup.dmg file you’ve just downloaded. Double-click on Uninstall. You Mac might ask your permission to open the app, choose Open.
3. Connect your Android phone to your Mac via the supplied USB cable. All content from your phone and memory card should show up and you can drag and drop to transfer files now.
Let me know if you have any problem resolving the issue.