block['content'] contail both string and form
drupal_get_form returns an array, so you can't concatenate it with a string.

Something like this will work, using a render array:

$block['content'] = [
  'preamble' =&gt; [
    '#markup' =&gt; '&lt;strong&gt;...&lt;/strong&gt;',
  ],
  'form' =&gt; drupal_get_form('custom_request_price_form'),
];