Contributor can make invoices & quotes

You could try something like the following:

function sliced_custom_params_admins_only( $opts ) {
	$opts['map_meta_cap'] = false;
	$opts['capabilities'] = array(
		'edit_post'          => 'manage_options',
		'read_post'          => 'manage_options',
		'delete_post'        => 'manage_options',
		'edit_posts'         => 'manage_options',
		'edit_others_posts'  => 'manage_options',
		'delete_posts'       => 'manage_options',
		'publish_posts'      => 'manage_options',
		'read_private_posts' => 'manage_options',
	);
	return $opts;
}
add_filter( 'sliced_invoice_params', 'sliced_custom_params_admins_only' );
add_filter( 'sliced_quote_params', 'sliced_custom_params_admins_only' );

Simply add this code to your (child) theme’s functions.php file, and it should do what you want.

It uses the filters ‘sliced_invoice_params’ and ‘sliced_quote_params’ to change the capabilities required — in this case, ‘manage_options’ is only available to admins.

I had entered the code into my child theme function.php file, and I’m seeing this message appear. Just want to make sure the code is correct before saving. Here’s a screenshot for you to review.

https://gyazo.com/90919ccf27f3f0b88ca6f228154994a2

The code I see in your screenshot definitely won’t work… all the single quotes (‘) have been converted into html entity codes. I would try pasting it into a plain text editor first (for example, Notepad), and then copy & paste from there to it’s final destination. It should look exactly the same as it appears above on this page.

…and if that doesn’t work, try copy and pasting from here instead: https://gist.github.com/grantalltodavid/b707a53dc2c1b10f4a9e425721dc23af

My bad. It works now. Thank you!