Not being able to sit in full lotus
Twenty years ago when I first started, I saw a Hinayana monk who was 84 years old sit in full lotus and his legs looked like spaghetti. He later lived until over ninety years old and never wavered.
In your case, if it is only a blood circulation concern around your knee, I would not worry too much. It is probably because you have never really tried to sit through the discomfort until your Qi is strong enough to break through the bend around the knee.
The secret in the full lotus position is to initially make it difficult for the blood to flow through the bend of the knee, thereby forcing your Qi to increase to resolve the issue.
Please understand that I am not qualified to give medical advice and that all of my students of all ages have successfully been practicing sitting meditation. Not all of them can sit in full lotus and yet they continue to make progress regularly.
In brief, I would like to recommend the following:
I wish you the best of luck and much bliss in your meditation practice.