If you are using the "Hyper Cache" plugin Your checkout page must be out of cache to work properly.

You need to exclude checkout pages by following these instructions-

Step 1: Note down the slug of your checkout page from the general settings

Step 2: Open the settings of  "Hyper Cache" under settings tab.

Step 3: Jump on the “Bypasses” tab.

Step 4: Scroll down the Bypasses page and locate for "Do not cache pages with URIs starting with" option

Step 5: Enable the checkbox option to use this setting

Step 6: To exclude “checkouts” pages enter your noted checkout page slug (from step 1) as shown in screenshot:  /checkouts/

Step 7: Scroll down the page and click on the "Save" button to update the settings.

By default, the Checkout page's slug is "checkouts". If you had changed the checkout pages slug for any reason get the slug from FunnelKit (formerly WooFunnels) > Settings > General > Permalinks