I am a new user and I have been successfully uploading to my ATmega2560 with a USBtinyISP. But now I keep getting the message...

  'Upload Using' requires a programmer. Upload cancelled.

I have "USBtinyISP" selected in the Programmer toolbar. I have "Enable Programmer" highlighted also (although I've tried it both ways). I have played with various settings in the Uploader menu.

The Arduino 1.8 IDE has no problem talking to the USBtinyISP programmer and uploading to my board. And as I said it worked up until this afternoon. I would provide some logs but I can't find anything really logged. I am at a loss for what else to try or where else to look.

Bob

Recall that even though an appropriate programmer was selected (USBtinyISP) and enabled, the VS extension still acted as if there wasn't a programmer selected. Also recall that it worked previously and that the standard Arduino IDE had no problem finding and using my programmer. That is why I felt that something became misconfigured in the VS extension. Looks like I was right.

I deleted and reinstalled the VS extension and that did not cure the problem.

I deleted and reinstalled the Arduino IDE (moving from 1.8 to 2 in the process) and that did not cure the problem.

Finally, I noticed that when the VS extension was uninstalled and deleted the Windows registry settings remained. So I deleted all the registry settings by hand with regedit and then reinstalled the VS extension. After doing that the extension was able to find the programmer and use it.

You may want to think about cleaning up the registry as part of your uninstall process.