Your site is missing a 512 × 512 icon, which allows users to add a home screen
shortcut to this site on Android devices.
Go to the Site Settings and upload a 512 × 512 icon.
I tried a random picture which is not 512 * 512, the problem remains.
I tried to download a 512 * 512 picture, but the problem still remains.
Are you sure it displays the same warning after you click refresh? On some installations it then also gives a warning about the short title not set or being too long, which can give the impression that the original problem remains, while in fact it’s a different one.