DHCP server is out of IP's to assign. Need to create a new expanded scope.

I don't think you can create two scopes with same start

use netsh to export scope info, modify according to new changes and import it back
Check step by step
https://community.spiceworks.com/how_to/120581-how-to-modify-an-existing-dhcp-scope-and-subnet

below is what I attempted to do.  Neither work.  I get the message in the file   "The command needs a valid Scope IP Address."

Microsoft Windows [Version 6.3.9600]
(c) 2013 Microsoft Corporation. All rights reserved.

C:\Users\Administrator.LASPCA>netsh dhcp server \\spcala234 scope 255.255.0.0 du
mp>c:\dhcp.txt

C:\Users\Administrator.LASPCA>netsh dhcp server \\spcala234 scope 172.18.1.12-20
0 dump>c:\dhcp.txt

C:\Users\Administrator.LASPCA>

see attached

You may need to expand your network if all addresses in the (SUB)network depleted.
If your current setup is 172.18.1.0/24   then you may need to use 172.18.0.0/23   ( which is 172.18.1.0/24 + 172.18.0.0/24 )
(effectively doubling the adresses.
(provided  172.18.0.0/24 is still free...)
Otherwise you may need to move to another range.

ok
make your changes with notepad ++ application

Below should work
https://it-reminders.com/2017/01/25/change-dhcp-scope-subnet-mask-and-others-settings-using-backup-and-restore/

Because this is the only way to use existing scope without deleting and recreating entire scope

Your scope seems to me to be misconfigured.
I am making an assumption that his is a 24 bit subnet with 254 address.
You have a scope that starts as 172.18.1.12 to 172.18.1.200 giving you 189 address to use vs the entire scope of 254 address. With this scope you are leaving out 54 addresses. Then you have a very strange exclusion for individual ip's along with a few broader exclusion.
With what you have posted you have 189 address with only 45 available address in you pool.

Could you expand on why you have a scope that starts at 172.18.1.12. but the first address that can be used is 172.18.1.144.  That alone excludes 132 addresses from your scope.  Then you have these one off exclusion.  You can accomplish this by using reservation for the addresses you want to assign to a specific device.

All in all this does not make much sense and whoever set this up seems to not have a solid understanding of subnets and DHCP.  

I would add another scope if available from 172.18.1.201-172.18.1.254.  This will give you another 54 addresses and if you can do a scope from 172.18.1.1-172.18.1.11 that is another 11 address.  All in in that would give you 64 additional addresses.

The reason I didn't use the other 54 is that I did not think our network would grow this large.

The reason I have the large exclusion is I was attempting to distribute the IP's over two different servers.  See screen shot.   This was probably not the best solution.  It was done many years ago.
I am open to suggestions of a better way to use two servers.  
Also since the IP's are assigned for an 8 day lease, it really would not be a problem deleting the scope and redoing both servers.

Suggestions welcome

What OS is this DHCP SERVER?

The one that needs expansion is 2012 R2, the other is 2008 R2

I am spinning up a 2016 DC as I type.   I will read the article.

The error you posted states that you have not altered the scope IP range in exported file with valid IP range or valid subnet mask
If you could correct that scope should get modified

So far I like yo_bee's, post.  Seems better for the future.

Since all the users' computers have a leased DHCP IP, I can delete both scopes one at a time and then configure Failover, correct?

correction to my last post.  I'll delete the 2008 scope built the new 2016, and modify the scope of the 2012 server.  
Then configure the failover.

Thanks to all.  This was my best solution.

Glad have help.  
For the future if you think you are never going to use something most likely you will at some point.