Error compiling for board ATtiny45

So I have to a senior project that a couple of classmates and I have to do. We are making a coffee mug that has a temperature sensor that changes the color of the LED according to the temperature of the cup. We basically used this website to get the circuit from. Now, we bought the parts from Jame Co. and the ATtiny45 chip we got is of course not programmed. (The one in their store does come with a coded chip but we want more than one chip and don't want to spend $20 per kit.) We do not have any coding experience at all and I have only touched Arduino for my Ambient lighting project on my monitor. I looked up how-to's and I got a good one from high low tech website. One problem that I found out is that the core that the high low website gave me is bad. It told me it couldn't find a serial for the Arduino, I later find out that ATtiny's don't use serials? Mmk. So, a person on Reddit gave me his core that he apparently had. I believe I installed everything right and when I try to even verify the code it gives me "Error compiling for board ATtiny45." I am seriously lost and have no idea what went wrong. Please help! If you need any more info, please don't hesitate to ask. Thanks in advance!

Try this core

Have not used Attiny45 myself.

Ya - my core should work for you, and if it doesn't, I will try to get it fixed ASAP if you report issues (via github issue is preferred - though if that's a hardship you can always complain around here)