This is caused by the ePay service account not having 'write' access to the location where the input file resides.
We recommend that this is done by somebody in your IT department who has full access to the server and is familiar with your IT infrastructure.
To find out where ePay is looking for your file, please see article: How can I find where epay is importing the input file from?
To find out which account is being used to run the ePay service, please see article: How can I find the account being used to run the ePay service (ePay user account)?
Once you have the above information please make sure the service account has 'read and write' access to the folder path and ensure the permissions on the shared folder are also setup to allow 'read and write' access. To check this account now has access. Log into the ePay server as the service user account and browse through windows explorer to the path used in ePay, make sure you can 'read and write' to this location.