Hello Ela
Yes, it is probably due to unawakened flexibility(!) that you cannot reach your foot.
Are you able to bend up your foot at all?
Here is a scenario that may apply to you:
Often with students (probably ones such as yourself) that are tight they can do only one of the 2 necessary actions:
1) they can either raise their arm up above their head
OR 2) they can bend up the leg to grab the foot.
If that is your problem then I recommend standing next to a wall during this pose so you can grab your foot and stand upright - balancing yourself with your other hand anywhere on the wall.
Kick your leg backwards just enough to get your bent up knee at least in line with, and hopefully behind the standing leg knee. You should then feel a very lovely stretch through the whole front side of the body from that bent leg thigh up through your torso. In time you will be able to lift your arm up high (even against the wall) and then one day (soon) balance without needing a wall.
If the above doesn’t describe you then try a modified grip, a strap or a towel:
One thing that you can try before using a small strap around your ankle is perhaps to hold your foot from the outside (just letting the arm hang down, palm facing in).
As you know the grip we are aiming for is with the hand coming from the inside. Technically speaking your arm externally rotates so that your elbow points into your waist and your palm is facing the side wall.
Please recognize that holding with a strap or from the outside doesn’t give you this beautiful opening through an external rotation of the arm. This will come in time.
Ela, hope that helps you. Keep asking if we didn’t hit the nail on the head! We’ll be eager to hear your progress!
Cheers!
Gabrielle 
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