LeanFT was introduced some time after ALM 12.20 had already been released. The functionality for LeanFT was not added to ALM 12.20 until some of the subsequent patches were released.
To get LeanFT as a test type in ALM you are going to need to get patched up to a newer version in ALM. I would suggest having your QC administrator patch ALM up to ALM 12.21 with Patch 5. Once your ALM is patched you still won't see the option for LeanFT as a test type, once the patch is in place you will need to have the QC admin run a ''repair'' on the project that you want to create LeanFT tests in, then once the repair has been run on the project then you will need to have the qc admin restart the ALM server. Once this is done then you should see the option for LeanFT appear in the test type drop down menu. The reason that you need to run a repair/restart is that this updates the project database with the new lean FT functionality that was introduced in the patch.