The luteal phase is the time from the day after you ovulate (1 DPO) until the day before AF starts. The typical length is 14 days (hence why they call it the 2WW), but it can be as short as 10 days and still be considered normal. Anything less than 10 days is considered to be a Luteal Phase Defect (LPD) making it difficult to conceive because the egg doesn't have enough time to implant before the lining starts to shed.
LPD can be treated OTC with Vitamin B6 or medically with Clomid or progesterone. Some ladies even use progesterone cream sold over the counter for menopasaul women, but I would urge caution before using as it has the potential to really mess up your cycle. Low progesterone should not be assumed, but diagnosed by your doctor.
Here is an article on LPD that gives greater detail:
https://www.babyhopes.com/articles/luteal-phase-defect.html
Hope that helps.