Once in the Print Module and once you have selected the Custom Package Layout, you need to make sure that you select the correct Custom Template from the Left Template Browser.
Please let us know if it helps.
Please reset the Preferences for Lightroom.
Go to C:\Users\User Name\AppData\Roaming\Adobe\Lightroom\Preferences (Appdata is a hidden folder)
Look for "Lightroom 6 Preferences.agprefs" >> Move that file to the desktop
Try again also, create new user preset for custom package.
Let us know if it helps.
Start with a blank page and one image selected and choose custom package.
Go the the Cells tab and click on a dropdown arrow and choose edit.
Enter 2.5 and 1.75
When you see your image with a blue bounding box hold down Alt and drag and keep repeating.
You will get duplicates of your first image but you can drag from the filmstrip to replace.