Hi there,
There is no way to correct this without the addition of custom CSS, which seems to not be possible under your current WordPress.com upgrade, but can be had by upgrading to the WordPress.com Premium plan. https://wordpress.com/pricing/
Your other option is to pick a darker button color. By default this theme makes the “hover state” of the button light than the current color. Because you have chose such a pale shade of purple, the hover state is so light it appears to be almost white, making the text invisible.
For future reference, your WordPress.com site also comes with built in support, so you are also welcome to ask for help here: https://wordpress.com/help/contact
This forum is for the self-hosted version of the Seedlet theme, which is a little different than the one that is included on WordPress.com, so it’s best to check with WordPress.com support as a first step. Hope this helps!