In my manifest file I have the following code:
<Requirements>
<Sets DefaultMinVersion="1.2">
<Set Name="ExcelApi" MinVersion="1.2"/>
</Sets>
</Requirements>
But it doesn't work, the manifest file is not valid.
The <Requirements> element has to be between the <Hosts> and <DefaultSettings> elements within a parent <OfficeApp> element.
We have not done a good job of documenting the required sequence. In the meantime, you can see the OfficeAppManifestV1_1.