Mount /data partition without losing data In some new models, when installing a custom recovery, /data partition won't mount. To fix that I need to wipe partition. How can I fix without losing data?
Android M added support for /data encryption, and Android N made it mandatory.

Old custom recoveries' having trouble mounting /data is primarily because they aren't capable of decrypting it. Newer recoveries will prompt for a password (or pattern) before they attempt to access /data (or nothing if you don't have a locked lockscreen).

Solution: Give up that recovery and get another one, but this time make sure the recovery you're installing supports /data decryption (like TWRP).