While it was previously possible to link your X account in order to automatically share your pixiv posts on X, changes in the specifications of the new Twitter API to be rolled out on April 29th, 2023, have retired this feature.
The checkbox feature "Share your work(s) on X" will be discontinued.
If you would like to share your work on X, please post the work to pixiv, then click the "Share this post" button.
You will still be able to use the "Tweet a thread" feature, with a few exceptions for certain works.
* Currently, no changes have been made to the feature allowing you to log in to pixiv through your X account, so this feature is still available.