Hi Ruotong Jia,
Thank you for reaching out to us on Microsoft Q&A forum.
Step 1: Try to fit the width on the screen. In the new relationship tab, it should work.
Step 2: In case you can't see the "OK" icon, try adjusting the 'Create Relationship' window. Kindly check the below screenshot as highlighted.
If you are still facing any issue, please let us know in the comments. We are glad to help you.
If the information is helpful, please Accept Answer so that it would be helpful to community members.
Thank you.