Can you post a screenshot of your version of instances.xml that you are using for your mod? My guess is a typo in your version of:

<Instance Name="MYNEWMAP_01_00_1916" Variant="MYNEWMAP_01_00">
<MapFileName> MYNEWMAP_01_00.ted </MapFileName>
<ArtServerObjectFileName> MYNEWMAP_01_00_SHARED.sob </ArtServerObjectFileName>
<LocationNameTextID> TEXT_NAME_MYNEWMAP_01_00_DMG </LocationNameTextID>
<LocationDescriptionTextID network="client"> TEXT_DESC_MYNEWMAP_01_00_DMG </LocationDescriptionTextID>
<PreviewLoadingTextureName>MYNEWMAP_01_00.tga</PreviewLoadingTextureName>
<IsDateVariation>true</IsDateVariation>
</Instance>

<?xml version="1.0"?>

<Instances>

<!-- *********************************************************** -->
<!-- *********** Skirmish maps ********* -->
<!-- *********************************************************** -->

<!-- Early War Version -->
<Instance Name="MYNEWMAP_01_00" Variant="MYNEWMAP_01_00">
<IsDeveloper> false </IsDeveloper>

<MapFileName> MYNEWMAP_01_00.ted </MapFileName>
<ServerObjectFileName> MYNEWMAP_01_00_ALLIED_CTRL.sob </ServerObjectFileName>
<ArtServerObjectFileName> MYNEWMAP_01_00_SHARED.sob </ArtServerObjectFileName>
<PassabilityFilePrefix>MYNEWMAP_01_00_SHARED</PassabilityFilePrefix>

<LuaMapScriptName>lua\MapScripts\MYNEWMAP_01_00.lua</LuaMapScriptName>

<!-- Even Map so this doesn't mean anything -->
<MPSkirmishAttackerFaction>Faction1</MPSkirmishAttackerFaction>
<MPSkirmishDefenderFaction>Faction2</MPSkirmishDefenderFaction>

<!-- Tier 2 is all toys and no bonuses : Should be overriden by Skirmish UI -->
<MPSkirmishTechLevel>2</MPSkirmishTechLevel>

<LocationNameTextID> TEXT_NAME_MYNEWMAP_01_00_DMG </LocationNameTextID>
<LocationDescriptionTextID network="client"> TEXT_DESC_MYNEWMAP_01_00_CLN </LocationDescriptionTextID>

<LoadingTextureNames>
<Entry>Empty.tga</Entry>
</LoadingTextureNames>
<PreviewLoadingTextureName>MYNEWMAP_01_00.tga</PreviewLoadingTextureName>

<RandomLoadingTextIDs>
<Entry> TEXT_CHAMPIONS_MAP_PVE </Entry>
</RandomLoadingTextIDs>

<MapFlagPosition network="client">
<X> 243 </X>
<Y> 243 </Y>
</MapFlagPosition>

<AlternatesByDate>
<Entry>
<StartYear> 1915 </StartYear>
<StartMonth> 0 </StartMonth>
<StartDay> 0 </StartDay>
<AlternateInstance>MYNEWMAP_01_00_1916</AlternateInstance>
</Entry>

<Entry>
<StartYear> 1917 </StartYear>
<StartMonth> 0 </StartMonth>
<StartDay> 0 </StartDay>
<AlternateInstance>MYNEWMAP_01_00_1918</AlternateInstance>
</Entry>
</AlternatesByDate>

</Instance>

<!-- Mid-War Version -->
<!-- Replace the MapFileName, ArtServerObjectFileName, and PreviewLoadingTextureName with alternate damage versions if wanted -->
<Instance Name="MYNEWMAP_01_00_1916" Variant="PUB_20x25_MAP_01_00">
<MapFileName> MYNEWMAP_01_00.ted </MapFileName>
<ArtServerObjectFileName> MYNEWMAP_01_00_SHARED.sob </ArtServerObjectFileName>
<LocationNameTextID> TEXT_NAME_MYNEWMAP_01_00_DMG </LocationNameTextID>
<LocationDescriptionTextID network="client"> TEXT_DESC_MYNEWMAP_01_00_DMG </LocationDescriptionTextID>
<PreviewLoadingTextureName>MYNEWMAP_01_00.tga</PreviewLoadingTextureName>
<IsDateVariation>true</IsDateVariation>
</Instance>

<!-- Late-War Version -->
<!-- Replace the MapFileName, ArtServerObjectFileName, and PreviewLoadingTextureName with alternate damage versions if wanted -->
<Instance Name="MYNEWMAP_01_00_1918" Variant="MYNEWMAP_01_00">
<MapFileName> MYNEWMAP_01_00.ted </MapFileName>
<ArtServerObjectFileName> MYNEWMAP_01_00_SHARED.sob </ArtServerObjectFileName>
<LocationNameTextID> TEXT_NAME_MYNEWMAP_01_00_SCR </LocationNameTextID>
<LocationDescriptionTextID network="client"> TEXT_DESC_MYNEWMAP_01_00_SCR </LocationDescriptionTextID>
<PreviewLoadingTextureName>MYNEWMAP_01_00.tga</PreviewLoadingTextureName>
<IsDateVariation>true</IsDateVariation>
</Instance>

</Instances>

I think the issue is either your first line: (and might be a combination of both)
<!-- Early War Version -->
<Instance Name="MYNEWMAP_01_00" Variant="MYNEWMAP_01_00">

where you have the instance being a variant of itself: MYNEWMAP_01_00
instead of a a skirmish instance (or another mid-step you created)
<Instance Name="MYNEWMAP_01_00" Variant="Base_SimpleRTS_Skirmish_Instance">


Or it might be an issue with your Mid-War version:
<Instance Name="MYNEWMAP_01_00_1916" Variant="PUB_20x25_MAP_01_00">

You have it being a variant of "PUB_20x25_MAP_01_00" instead of your new instance: MYNEWMAP_01_00 - since I am guessing you don't have the Sample Skirmish mod loaded at the same time - it can't find PUB_20x25_MAP_01_00 so is complaining (or will when you try and load the mid-war version in game).

Your late-war version is a correct variant of your base:
<Instance Name="MYNEWMAP_01_00_1918" Variant="MYNEWMAP_01_00">

I just started from scratch and it seems to be working... Sorry this is my first mod experience. But I am SO excited to learn. Ive been playing with the map editor and this is a perfect middle aged man hobby.... making scale battlefields

no problem (lot of places to copy and paste into!) - glad it is working for you!
look forward to trying it out!

Did you write the manual? Because if so you did a great job. (IE pg 61 at the very top when you explain how selecting an obj then fills in all the info on the side bar).

Joint effort with a few other people (but a lot of initial PG_Ren work)
Glad you are finding it useful - always a worry between too much detail and not enough information!