WHMCS Error:

Error: Call to undefined method WHMCS\Http\Client\HttpClient::requestAsync() in

Rename vendor/ and modules/ folders

root@server [/home/user/public_html/hosting]# mv vendor/ vendor_old
root@server [/home/user/public_html/hosting]# mv modules/ modules_old

Upload fresh appropriate WHMCS version files into temp folder, for example: whmcstemp/

Copy vendor/ and modules/ folders from whmcstemp into your WHMCS installation folder:

root@server [/home/user/public_html/hosting]# cp -rp ../whmcstemp/vendor/ ./
root@server [/home/user/public_html/hosting]# cp -rp ../whmcstemp/modules/ ./

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