SQL Server 2012 cannot use file server for log shipping
Make sure your log shipping primary instance's SQL Server and SQL Server Agent services are configured using a domain account and has write access on the specified backup share. If these are in place it should work fine.

Note: The SQL Server Agent can be configured to use a local account as well. The primary issue is that whatever account is used must have read/write access on folder where backup is happening.

To check it manually, you can try to take a manual backup on the share specified to see if that works and make sure write access is in place. I am assuming that your SQL Server and SQL Server Agent accounts are configured to use the same domain account. 

Run the below commands. Just change the DBName and FileserverPath as you have in your environment:

BACKUP DATABASE [DBName]
TO DISK = N'\\[FileserverPath]\DBName_FULLBackup.bak' 
WITH INIT;
GO
EXECUTE sys.xp_cmdshell 'dir \\[FileserverPath]\';


If this gives an error, create a folder on your fileserver, share it and grant write permissions to the SQL Server Agent account, then try again.