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Hi, same thing here.
I’ve tried using twenty sixteen theme and disabled all other plugins except woocommerce and pos. It works on older version of woocommerce, but not with the newest (3.6.4). In both situations I get this error:

[error] 2675#2675: *9129 FastCGI sent in stderr: "PHP message: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /wp-includes/class-wp-hook.php on line 286" while reading response header from upstream, client: xxx, server: xxx, request: "POST /wp-json/wc/v3/orders HTTP/1.0", upstream: "fastcgi://unix:/var/run/php7.2-fpm-xxx.sock:", host: "xxx", referrer: "xxx/pos/"

Woocommerce 3.6.4 change log says this:

* Dev – REST API – Stop order creation if invalid data is sent. #23325

Maybe these things are connected.

Please help