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
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Step 2: Open the settings of "Hyper Cache" under settings tab.
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Step 3: Jump on the “Bypasses” tab.
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Step 4: Scroll down the Bypasses page and locate for "Do not cache pages with URIs starting with" option
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Step 5: Enable the checkbox option to use this setting
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Step 6: To exclude “checkouts” pages enter your noted checkout page slug (from step 1) as shown in screenshot: /checkouts/
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Step 7: Scroll down the page and click on the "Save" button to update the settings.
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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