The solution is to add the following 2 lines of code in the file classes/WC_AlphaBank_Gateway_Base.php:
'extInstallmentoffset' => null,
'extInstallmentperiod' => null,
just after the line 'trType' => $trType, and just before the line 'confirmUrl' => get_site_url() . "/?wc-api=WC_alphabank_Gateway&result=success",.
