Sorry, The screenshot doesn't have complete steps I took...
Here are the steps I took from the beginning
Opened SSH and then run these command
cd
cd<space>public_html
dir
then a list of directory shown
rm<space><hyphen(-)>r<space>forum
rm -r forum
mkdir forum
cd forum
composer create-project flarum/flarum . --stability=beta
after this the error occur as shown in the above image