This is the 1590th most frequent Russian word.
сотрудничеству
"Сотрудничеству" means "cooperation" in English.
The word 'сотрудничеству' in this sentence is used in the form 'сотрудничества' as a noun, referring to the general concept or practice of collaboration needed for the project.
Этот проект требует эффективного сотрудничества.
This project requires effective collaboration.
The word 'сотрудничеству' is used in relation to 'к долгосрочному', indicating the direction or goal of establishing cooperation.
Мы стремимся к долгосрочному сотрудничеству с вашими партнёрами.
We are aiming for long-term cooperation with your partners.
'Сотрудничество' is employed here as an expression of gratitude for the cooperation received from someone.