I can’t save custom menu…

It’s a common WordPress problem. If you uploaded the themeforest folder with it’s original name (For example; /wp-content/themes/themeforest-7517792–sportyresponsive-wordpress-theme-for-sport-clubs ) the folder name may be too long for wp and it can’t save the theme location.

I’d suggest to rename the theme folder from themeforest-7517792–sportyresponsive-wordpress-theme-for-sport-clubs to sporty (or any other unique word) to save custom menu and to avoid any other issues.