"FILE MAX" is displayed when I try to use the USB recording function.

Recording is not possible if there are 999 recorded files in the "PIONEER REC" folder on the USB device or a file named "REC999.WAV".
You can record again after performing one of the operations below.
(1) The maximum number of recorded files is 999, so if you already have 999 recorded files, use your computer to delete some recorded files or move them from the "PIONEER REC" folder.
(2) If there is a file named "REC999.WAV" in the folder, change the file name.