Does ZTE prevent the use of adoptive storage with Marshmallow? I was trying to format a 32 GB Samsung SD card for adoptive storage in a ZTE Z815 running Marshmallow, but the only option is portable. What's going on here?
Your device doesn't have this feature enabled by OEM . Similar to other models as you can see from Google search and for another ZTE model Convert portable SD to Internal SD where option is unavailable

Same is the case with some Samsung devices. You would need to search at places like XDA for workaround like this

Also see this question for workarounds 

How can I move apps to SD on non-adopted Marshmallow storage?