Here are some steps to try in order to approve or reject a document in a SharePoint workflow:

1. Check if you have the sufficient permissions to approve or reject the document.
2. Check the steps in the workflow to make sure the document has moved through all the necessary steps.
3. Check the version of the document in the workflow to make sure that the version you are approving or rejecting is the latest.
4. Check the status of the workflow. Make sure it is not paused, suspended, or incomplete.
5. Verify if the required date and time fields on the document have been filled in.
6. Verify if all the required fields for the document instance have been filled in.

You can also try to recheck and approve/reject the document from different browser.