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