If you are trying to set up your ATtiny board and using the Digistump Board Manager, you might encounter this error. It seems Digistump has dropped support for this ( even their site appears not to be working anymore )

To fix the above, we will use another board manager that is also open source and based on the Digistump

Open preferences/settings

Paste below in the Additional boards manager URLs

https://raw.githubusercontent.com/ArminJo/DigistumpArduino/master/package_digistump_index.json

Now open the board manager from tools -> boards -> board manager

Search for DigiStump Boards and install it

Now you can program your Attiny85

PS: If you are on Windows and have not set up the drivers for Attiny yet, please download the drivers from here

Unzip it and run the install drivers script