I have high blood pressure
I have a high blood pressure
Is not correct.
We do not use the phrase “a pressure“, unless (1) you are talking about a specific pressure at a specific location,
I feel a strange pressure in my belly.
The air pressure gauge shows a pressure of 30 psi.
Or (2) you are using the article “a“ to introduce something.
I feel a pressure to conform.
I feel a growing pressure to conform.
I feel pressured to succeed.
You can say:
I have a high blood pressure level.
This is all right, because you’re saying “a level”, not “a pressure.” This is like saying:
I have a high level of blood pressure.
If you mean to say you have a specific reading which is unusual, you can say:
I have a high level of blood pressure right now.
My blood pressure is high (right now).
My blood pressure is higher than usual.