Make part Flexible - dont even have an icon

Hi, I Have genuine Solidworks professional 2020 SP5.0 and have an assembly where I would want to have a rubber boot flexing to simulate suspension travel of my components.

However I have watched plenty of youtube tutorials of how to make part flexible, and realised I dont even have that icon that is usually located after copy part appearance that looks like a cross with arrows in front of part icon.

I also right clicked on the component (rubber boot) that I want to make flexible and under component properties there is grayed out option for making component flexible.

Does anybody know how can I activate that option / feature and what am I missing?

Thank you in advance

Best regards,
Luka Jelaska