"are you sure you want to do this" error when trying to upload a theme


I recently purchased the theme “Gastro” by Twisttheme.

When I try to install the theme via wordpress I get the error "are you sure you want to do this"
And if I install the theme via ftp, when I install it I get completely locked out of my domain with an html error 500.
So had to completely uninstall everything from my server and start again. Luckily it’s a brand new website with no content yet.

I contacted my server provider and they suggested it may be a php memory issue, so I increased the memory to 128mb. Still having the same issue.

Server stuff isn’t really my area so any assistance would be greatly appreciated. The theme developer appears to have shunned me, saying it’s a server issue.

I resolved this issue.

I had to increase the php memory and also increase the post_max_size memory.

Unfortunately rose to further issues with the theme. As soon as activated I started getting the html error 500 again.
According to server logs the issue is:
PHP Parse error: syntax error, unexpected ‘[’ in /var/www/vhosts/salt-cafe.co.uk/httpdocs/wp-content/themes/gastro/includes/gastro-helper.php on line 506

Anyone able to assist? I am not a php developer.
Line 506 seems to be this line:
$image_url = !is_numeric( $args[‘url’] ) ? : [0];
I can’t see anything wrong with it.