NSX does not support snapshots. The best thing you can do is to disable them. To do this you need to power down the appliance.
In the vCenter shut down the appliances and then edit the settings of the NSX appliances, head to VM Options and Configuration Parameters field, click edit to add a parameter.
Add snapshot.MaxSnapshots with a value of -0 and click OK at the bottom of the Window.
To check it you can try to take a snapshot. It should give you an error like this:
This of course brings me to one other topic. Backup.
Backing up the settings
So since we just disabled the ability to take snapshots you can not use snapshot based tools like Veeam to take a backup of your appliance. So instead what you need to do is to to do a backup schedule inside NSX and the back these files up from file level. That means that if/when your NSX breaks to restore it you have to redeploy the appliances and then restore the configuration that way.
To do this go to System –> Lufecycle Management –> Backup & Restore
As you can see the automated backup is disabled but hitting the edit button allows you to set this up. note you need an sftp server for this.
On the second tab you can schedule your backup pretty granularly. Default interval is every hour. Alternatively you can do the schedule on a weekly basis on a set hour. There is also a feature to enable change detection. This, if enabled will mean a full unscheduled backup is performed when NSX detects a change has been made.