Hi,
This error is probably a conflict between plugins, so it would also be helpful if you could run a plugin conflict test to find out which plugin or theme might be conflicting with Fluid Checkout.
Basically, you temporarily deactivate all plugins except for WooCommerce and Fluid Checkout, then re-activate each plugin one by one until you find one that causes the issue. Ideally this should be done in a test/staging copy of your website. You can find more specific instructions in the article below from WooCommerce:
https://docs.woocommerce.com/document/how-to-test-for-conflicts/
Best,
Alan.