Product is not showing up in the object list when I am trying to create a master-detail relationship from my junction object.
what's the workaround?
Product can show up as Product2 in the API interface. The Product2 object does not actually support being set as the Master in an M;D relationship,but I can't find any specific Salesforce documentation that addresses this.
You can always use a Lookup relationship instead of a Master-Detail. There is a limit of 2 Master Detail relationships on a specific custom object so that may be blocking you, otherwise the following may apply:
You can't create a master-detail relationship if the custom object already contains data. You can, however, create the relationship as a lookup and then convert it to master-detail if the lookup field in all records contains a value.
This previous question and answer may also help.