Plase use this snippet
define( 'BP_ENABLE_MULTIBLOG', true );
Put it on "bp-custom.php" in folder "wp-content/plugins". If you dont have it please create a new file. After put the snippet, please clear your website cache.
And then, please add this snippet also on same file if you want to enable multisite Blog system
https://gist.github.com/KaineLabs/023c4fcea9de2703b06b94dfd4f40e0e