At certification.bubble.io, make sure you're signed in using the correct Bubble account.

Go to your profile by clicking the account circle in the top right.

Under your certificate link for the Bubble Developer Certification, click the button to add your certificate to LinkedIn.

This will take you to LinkedIn. Once you’re signed into your account, a popup will open with your certificate’s information pre-filled. Hit “Save” to add your certificate to your profile.