What could be causing this error?
Commerce Stripe is a bit of a mess at the moment.  There are lots of issues that need to be fixed as you can see in the project issue queue.
You are probably experiencing the bug described on Stripe\PaymentIntent instance has invalid ID, but there is a longer discussion about how to handle this at [meta] SCA, the Stripe Review pane and recurring payments.
Basically, the issue is that Commerce Stripe does not generate a payment ID unless you go through the review pane first.  That means that if you configure a standard Commerce checkout flow and go through that flow (login -&gt; cart -&gt; checkout, etc.), then the module will probably work.  If not, you will spend hours searching the issue queue to try to debug the module.
Source: Have spent several frustrated hours over the past couple years with Commerce Stripe.
If it helps, I'm currently using the following patches just to get the module to work on my site:
    &quot;drupal/commerce_stripe&quot;: {
        &quot;do not load stripe library on every page&quot;: &quot;https://www.drupal.org/files/issues/2020-02-03/3083393-21.patch&quot;,
        &quot;skip review step by autosubmit&quot;: &quot;https://www.drupal.org/files/issues/2020-09-16/commerce_stripe-3171411-2.patch&quot;,
        &quot;fix 1112 errors https://www.drupal.org/project/commerce_stripe/issues/3177168&quot;: &quot;https://www.drupal.org/files/issues/2020-10-16/3177168-2_commerce_stripe-fix_address_errors.patch&quot;,
        &quot;fix TrustedCallbackInterface warning https://www.drupal.org/project/commerce_stripe/issues/3168822&quot;: &quot;https://www.drupal.org/files/issues/2020-10-09/PaymentMethodAddForm%20needs%20to%20implement%20TrustedCallbackInterface%20for%20Drupal%209-3168822-14.patch&quot;
    },