I ran into this problem after replacing the main board of my Asus ME301T tablet. I successfully installed TWRP but it couldn’t find my backups that I have saved on SD card. Being impatient, I just reinstalled Lollipop, all my apps and restored app data with the Helium app.
Researching this afterwards, I found this youtube video.
The problem is that TWRP creates a subfolder under BACKUPS based on the device’s serial number. By changing the main board, the serial numbers no longer match. The trick here is to do a new small backup (backup the cache only) which will create a new backup folder with the device’s new serial number. Write down the name of it, delete it and rename your old backup folder the same name. TWRP should now see your backup files. You can perform all these file operations using the file manager in TWRP.
The device’s serial number can also be found under Settings->About Tablet->Status