Use @ for links.

src/Form/FillPdfSettingsForm.php

@@ -48,8 +48,8 @@ class FillPdfSettingsForm extends ConfigFormBase {
 
     // Assemble service options. Warning messages will be added next as needed.
     $options = [
-      'pdftk' => $this->t('Use locally-installed pdftk: You will need a VPS or a dedicated server so you can install pdftk: (<a href=":link">see documentation</a>).', [':link' => Url::fromUri('https://www.drupal.org/docs/8/modules/fillpdf')->toString()]),
-      'local' => $this->t('Use locally-installed PHP/JavaBridge: You will need a VPS or dedicated server so you can deploy PHP/JavaBridge on Apache Tomcat: (<a href=":link">see documentation</a>).', [':link' => Url::fromUri('https://www.drupal.org/docs/8/modules/fillpdf')->toString()]),
+      'pdftk' => $this->t('Use locally-installed pdftk: You will need a VPS or a dedicated server so you can install pdftk: (<a href="@link">see documentation</a>).', ['@link' => Url::fromUri('https://www.drupal.org/docs/8/modules/fillpdf')->toString()]),
+      'local' => $this->t('Use locally-installed PHP/JavaBridge: You will need a VPS or dedicated server so you can deploy PHP/JavaBridge on Apache Tomcat: (<a href="@link">see documentation</a>).', ['@link' => Url::fromUri('https://www.drupal.org/docs/8/modules/fillpdf')->toString()]),
       'fillpdf_service' => $this->t('Use FillPDF Service: Sign up for <a href="https://fillpdf.io">FillPDF Service</a>.'),
     ];