A record is being edited in FileMaker, i.e. the cursor is in a data field or a data field is activated.

If a second window is now opened that accesses the same data table AND the same record, then this record cannot be edited here. When you try, FileMaker sends the message "This record cannot be edited in this window because it is already being edited in another window."
This is because FileMaker has locked the record for further editing, possibly by another user.

In the "Window" menu you can see which windows or databases are open in FileMaker. You can also select the windows here. You may also have to look in the sub-menu "Show window", here you will find the windows that are in the taskbar.
These windows are sometimes opened automatically by scripts, so users do not always have to be aware that a database is open in several windows.

To be able to finish editing the record in question in one of the windows, one must select and close all other windows that have the same record being edited.

After all other windows have been closed, the data set can be finished in the only window that is still open.
By the way: It is not absolutely necessary to close the windows. It is sufficient if the record is no longer being edited in any other window. Therefore, no data field may be activated there for the data set in question.