Please contact the appropriate software vendor or IT professional for assistance with this process or issue, which is beyond Blackbaud's scope of support.

The Windows account being used by the SQL Server Service must have sufficient permissions on the destination folder and/or drive.  For additional information on resolving this error, refer to the following Microsoft Support article:

You cannot back up database to a network drive if your account have not sufficient permissions to access the network drive
support.microsoft.com/kb/207187