There are only 2 reasons behind template not being available in dropdown of email nodes in journeys or Journey 2.0 though created on the panel. Below are the reasons :
1) For email templates to be used in journeys, it is mandatory that images being used in the template should be hosted images. This means, you are supposed to host those images on your server or using 3rd party websites and then use those hosted links for the images in the HTML.
If the images are inline i.e not hosted, the template wont be available in dropdown of email nodes in journey.
2) In Journey 2.0, digestification is not supported. So if there is a digestified template which was being used in old journey builder and want to use it in Journey 2.0 as well, then it will not be available in dropdown.
You will have to create template using product collection. Refer https://docs.netcoresmartech.com/docs/email-personalistion#product-collection-in-emails-through-journey-20 for complete understanding on Product collection and steps on how it can be used.