If you get an error message Download failed. A valid URL was not provided. during plugin installation then It sounds like you are trying to install a plugin using TGM Plugin Activation from the child theme. In this case please:
- Temporary switch your theme to parent theme (WP Dashboard -> Appearance -> Themes) and then install recommended/required plugins.
- Switch back to your child theme and then activate these plugins again.
Installation and activation should work fine this way.
Notes: This is related to TGM Plugin Activation.