I'm using slightly different software versions, but this just works:
mkdir /dev/shm/bitcoin; bitcoin-qt -datadir=/dev/shm/bitcoin
It saves on /dev/shm (which is more or less a RAM-drive in Ubuntu). Can you test this, just to see if that gives you the same error?
the only way to make the bitcoin-core to work is to let download the block chain to the default directory but that's not what I want.
Just in case: In your screenshot, you ticked pruning. That means it'll fit on the default location too.
I wanna download the all block chain that's why I got the 500gb hdd
but my question is why when I click the "..." to choose the download directory the "computer" "recent places" are grey and no directories are display. I install ubuntu 19 more then 3 times and also version 18 and the same stuff I see the same grey "computer" check out the video and you will understand exactly what I mean
https://streamable.com/x1nctIf nothing else works, another idea could be to make a symlink with the name Bitcoin expects and which points wherever you want.
I did , same error the only way that the bitcoin-core can work if I let it download the blockchain to the default partition.