When in doubt, a condolence message should be simple and succinct. Below is a list of suggestions I have recommended to people:
I’m so very sorry for your loss.
My thoughts [and prayers] are with you during this difficult time.
Sending you love and comfort during this difficult time.
I/We love you.
I am/We are here for you.
My/Our deepest condolences.
I hope you can take comfort in the love you shared and the memories you created.
My heart breaks for you. I’m so sorry for your loss.
I will be holding you in my heart while you grieve this very difficult loss.
Wishing you peace during this difficult time.
Your memories together will live on.
I know how much [they] meant to you. [They] will be deeply missed.
Surrounding you with love and comfort.
For more ideas, Get In Touch and I will help you brainstorm.