Hello @SKUMA.1 and welcome to the Community and STM32 word :)
At the moment mbed supports only NUCLEO-H743ZI2 and DISCO-H747I boards.
You may want to use the mbed compiler with one of this H7 board instead STM32H735.
Otherwise, I advise you to have a look at this link, which provides the steps on how to create/add a new STM32 platform.
You can also review this doc, to create a configuration for a custom platform.
Hope this helps!
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Imen
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