Hi there,
that would mean either a security plugin or security mod on your server is stopping those from being uploaded.
If you use no security plugin then you would need to check with your host.
Alternatively, unless you need very specific TTF styling options you may want to consider just loading .woff and .woff2 fonts as they are supported across all modern browsers.
And if you only have ttf fonts you can use a font convertor like the one at Font Squirrel to convert them