Error when building for the first time

Hi guys, so I working in mozilla Thunderbird for a school project and when building, it gave me this error:
checking for hg… not found
0:06.99 DEBUG: hg: Trying ‘C:\Program Files\TortoiseHg’
ERROR: Cannot find hg
I have nuked the repo twice now and searched the internet for this error but I could not find anything related to this error. I have used both source tree and hg clone to download the mozilla core and the thunderbird repo. Any assistance is appreciated.