ios plugin error commuciating with device

[broden]

I have a iPad pro and itunes 12. Calibre reports the following error.
calibre, version 2.57.1
ERROR: Error: Error communicating with device

(-2147352567, 'Exception occurred.', (0, None, u'The track is not modifiable.', None, 0, -1610350077), None)

Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 87, in run
File "site-packages\calibre\gui2\device.py", line 584, in _upload_books
File "site-packages\calibre\devices\apple\driver.py", line 1234, in upload_books
File "site-packages\calibre\devices\apple\driver.py", line 3130, in _update_iTunes_metadata
File "site-packages\win32com\client\dynamic.py", line 560, in __setattr__
com_error: (-2147352567, 'Exception occurred.', (0, None, u'The track is not modifiable.', None, 0, -1610350077), None)

Am I doing something wrong?

Any assistance appreciated.

Up date in fact .PDF files are copied to the iPad but without the title. File are on the iPad with the apple internal name instead of the files name.