Hello,
this message only says that the PDF could not be created when saving. It does not include the reason, so unfortunately we cannot tell from it what went wrong.
Since your post we have fixed several problems in the form editor, among them some with copied fields. Please try again: online at https://tools.pdf24.org/en/make-pdf-fillable (the online tool now also shows the reason if saving fails), in the PDF24 Creator the fixes will come with the next version.
If the error still occurs, please tell us whether you use the online tool or the PDF24 Creator and which message appears. So that we can reproduce this, please send us a file where it happens to forum@pdf24.org