The problem happens following the doc and testing the SP

Specifics of your environment

1. Acting as SP
2. SimpleSAMLphp: 2.3.2?
3. PHP: 8.2
4. Platform: ubuntu 22
5. Webserver: Apache

Describe the bug
The documentation here
https://simplesamlphp.org/docs/stable/simplesamlphp-sp.html
describes the idp as

<?php
$metadata['https://example.org/saml-idp'] = [
    'SingleSignOnService'  => 'https://example.org/simplesaml/saml2/idp/SSOService.php',
    'SingleLogoutService'  => 'https://example.org/simplesaml/saml2/idp/SingleLogoutService.php',
    'certificate'          => 'example.pem',
];

but if you do this with a Shibboleth (and I guess others Idp), simplesaml is complaining about having a string and not an array for SingleSignOnService, SingleLogoutService.
In fact, when you go deeper on the doc
https://simplesamlphp.org/docs/stable/simplesamlphp-reference-idp-remote.html
You can see the SingleSignOnService entry that points on https://simplesamlphp.org/docs/stable/simplesamlphp-metadata-endpoints.html
and on endpoints format you have the proper format, ex :

  'SingleLogoutService' => [
      [
          'Location' => 'https://sp.example.org/LogoutRequest',
          'ResponseLocation' => 'https://sp.example.org/LogoutResponse',
          'Binding' => 'urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect',
      ],
  ],

So, in the first page, the doc is wrong but I don't know the format with simplesaml as i'm using shibboleth.
I solved it using the xml idp metadata converter which is the right tool to use. I suggest to suggest in the doc to use it first if the users already have a working IdP, it could save hours and debugging the error in sp testing that doesn't explain anything.

If you gimme the proper format i can pull request for you.

Thanks !