Explanation/Reference:
Note:
* RangeNameis one of the following values:OldVersions,NewVersion,AllVersions.
Here we must NewVerssion as " the web application must be able to create sites only with a compatibility level of SharePoint 2013".
* The last step is to validate.
* At the Windows PowerShell command prompt, type the following command to change the compatibility range settings to a specific range:
$wa=Get-SPWebApplication<URL>
# Stores the web application at that URL as a variable
$wa.CompatibilityRange = [Microsoft.SharePoint.SPCompatibilityRange]::<RangeName>
# Specifies which range to use
$wa.Update()
# Updates the CompatibilityRange setting to use only the range you specified
$wa.CompatibilityRange
# Returns the new CompatibilityRange for the web application
Where:
<URL> is URL for the web application that you want to change.
RangeName is one of the following values: OldVersions, NewVersion, AllVersions.
Reference: To change compatibility range for site creation modes for a web application by using Windows PowerShell