because it was created by a different version of the server (14.00.3411) than this server (14.00.3421).
According to the message your current version of the server is Cumulative Update 27 and your last backup was done before the update. In order to use that the old backup you will need an instance of that version and probably rebuild all the system databases to that version.
The problem is, we don't have the install binaries for SQL Server 14.00.3411 (CU 26) and cannot download version 14.00.3411 [what it says is needed].
What should have been done if you managed the server in the right way is to have a backup after your upgraded the server. Well... this is for next time :-)
I will describe what I would probably do (at least start to do and think as I go), but anything you do is on your responsibility as this is only the basic information and it is best to do it with an expert near you.
First of all, I would backup all the user database!!!
If we can login the server and use the BACKUP command then create full backup of all the databases (system and user).
If the server is not available then stop the service and backup the files (all of them)
Remember to check the backups before moving forward! A backup which was not tested is not a backup which we can count on.
Next there are two approaches
Option one: start from new and upgrade to CU26
if you have backups of all your other system database from the older version and you are OK with losing any information between these backup and now, then we can install new instance -> update to CU26 (Erland gave you the link) and restore all the system databases using the backups -> next we can bring the user databases.
This is complex and long...
Option two: simply uninstall the latest CU and downgrade to CU26
I have no idea what is corrupted in your databases but in theory this is a simple solution, which does not require any old file of CU26.
It is well documented in the document of CU 27 just like in most CU upgrade document you have information about how to go back:
Scroll down to the title: How to uninstall this update on Windows
Note! MAKE SURE THAT YOU DO NOT UNINSTALL THE BASE INSTALLATION OF THE SERVER BUT ONLY THE LAST CU. If you select a wrong file then you might uninstall the entire server, which mean that next you go back to option one.