Open Facebook and go to the post you want to make shareable
Click the three dots in the top-right corner of the post
Select Edit audience or Privacy settings
Change the audience to Public
Save the changes
Check that the post is not set to Only me, Friends, or a custom restricted audience
If the post is in a private group or on a private profile setting, change the privacy settings to allow public sharing
For future posts, choose Public before posting
If the post is in a Facebook group, make sure the group allows sharing
If the post is on a Page, ensure the Page post is public by default