Welcome to the forums.
You have clearly looked up the word, so which one do you think is the one to use?
A boss of my partnership companies passed away recently so I'm going to write a message to the company. but I'm confused what word is right between condolescence and condolence. my dictionary does not say "Condolescene"
But I have a question, usually I visit to Google Books to check spelling whenever I am confused. the website posts published books that were corrected by speller. so most of contents could be reliable I think.
I can find several examples on the website with Condolescene even though the website does not say it can't find the word
condolescence - Google Books
Mind you, most of those texts using 'condolescence' are not by native speakers. I'd also use the plural and offer my condolences.