The top of my mattress is 26" off the floor. It's a platform bed and the mattress was designed to be used without a box spring (can't be flipped). I like the height. I built the platform myself.

26" off the floor is fine, but does it interfere with the bookcase headboard?

I guess I am a little perplexed about what you are inquiring about specifically. You asked for advice and what to buy, but it sound like you purchased the items already. Can you give some additional direction on what feedback you are looking for?

I have the headboard , the box spring and the mattress , both the box spring and the mattress height would be 14.5" , so should I buy panel bed or platform bed without the headboard, which one I should buy and what is the difference between them and to a maximum of 24", how much height I should buy the panel or platform bed either one.

Panel beds typically have a bed rail, which you'd want to use the box spring. Platform beds are usually made that a box spring isn't needed, but if you really wanted to put a box spring on, you still could use one. I've seen platforms using slat systems that are adjustable height so you can set it a couple of inches higher or lower. Some platform beds have storage drawers already included; you could make or purchase faux drawers for under a panel, so you have storage options for that if you should chose.

Really you could go either option. Do you have a photo or link to the head board that you have?

That is what exactly I have, I put photo with the dimensions too, thanks

It looks like your head board has brackets for a frame with the rails.

Yes, what do u think ?

A) You can purchase the frame intended for the head board (rails); you will need to use the box spring

B) build or purchase a plat formbed, which you likely can forgo the box spring if you wish; if the head board is not free standing, particularly if it is not going to be sitting against a wall, you would want to figure out how to modify the head board to attach/secure it.

The headboard can stand by its one, so if I want to buy a platform bed, I should have accurate measurements so it would fit into the headboard , is that right?
Before I come Canada, we just bought bed all included , I didn't recognize that to buy a bedroom set we have to buy all the pieces separate like that, I think that is mix and match is a good thing but I am just beginner and I am confused a little pit ☺

You can use the dimensions of the head board to purchase a platform bed if you prefer a platform bed. You would have to measure the platform bed to know if it will fit "in" the head board (as in slide between the legs/sides of the head board) or not. If you do find one that will, you'll mostly likely need to not add the rail brackets when you assemble the head board. Likely, though, the platform bed base will set against the head board rather than "in", but that is something you'll have to check as you go to purchase the bed you want. Alternative, if you're handy, is you could build a platform to your specifications.

I wish I can DIY but I am not handy at all lol
Thank you for everything

First, when you buy a mattress and box springs, the springs are essentially the sleeves from the manufacturers VEST. They have zero value. You paid for a mattress.

Second, the headboard you purchased as stated above , is meant to be used with a "hollywood" , cheap, ordinary metal frame. Usually around 50.00 bucks.

If you want to use it with a PLATFORM, buy something like this below, and have a handy person attach /bolt to your headboard. I doesn't need the box spring.

Otherwise, I'd suggest you buy a wood bed that does exactly what you want, allows the use if a box spring, which will be hidden by side and foot rails that are finished. Both concepts below

Or you really want something like the entire bed, to allow the box spring

can you substitute wooden planks instead of the box spring on the panel bed?