web site crash after installing amazon pay

I have a problem with the plugin for woocommerce Amazon pay.
the web site is https://www.etnaorange.com/

I have spoken with amazon, they have checked and they say that for them everythink is ok, so they say that it is up to the team of Woocommerce to solve the issue.

As soon as I add the Merchant account into the field of the plugin, the web site crash:
A critical error has occurred on your website
Learn more about debugging in WordPress

The Woocommerce version installed is 3.9.4
The plugin Amazon pay version is 2.0.3

So right now I have removed the merchant account number, in order to let the website working

please write to me for solve this issue.
my amail is: [email protected]

Thanks in advance
Claudio

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