Where do you miss the button? I can see it correctly (iOS app).

Note - an activity needs to be public to be share-able.

Yes I use IOS. My activities are not private for my friends. I want to share my activity on instagram but i cannot find the share button.

Try setting the activity to public (everyone). The sharing button will then appear under the activity (next to the kudos 👍 and comments 💬 buttons).

Now everyone can see my activities but i still not can share anything 

Can you post a screenshot perhaps?

The button should be in the bottom right.