Issue on uploading a file by using web service

Hi,

I'm using Moodle version 3.8 and by webservice function core_user_update_users I'm uploading a file.  I used custom plugin for upload a file from user profile.

Now we've two instances, first instance successfully uploading a file while second instance showing below error.

Note: Same error were showing on first instance but after purging a cache it was resolved, but on second instance the error still persistent. 

{"exception":"Error","message":"Call to undefined function file_save_draft_area_files()"}

Thanks!