This indicates a permissions issue. The only two workarounds are:
1. Use your hosting control panel's file manager
2. Ask your host to do fix the ownership and permissions of all your files and directories
I strongly recommend pursuing the second workaround, even if the first one works. You will need to do that before trying to reinstall Admin Tools on your site.
For more information regarding ownership and permissions (which is the root cause of your failed Admin Tools installation) please consult Akeeba Backup's
Security Information chapter. In fact, I suggest that you read and learn this material today. Without this basic knowledge you can not manage your site successfully and every bit of advice I will attempt to give you to help you will sound like Greek to you.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!