In app/Database/Migrations/20170502221506_sales_tax_data.php on line 277 (in round_number()):

elseif ($rounding_mode == Migration_Sales_Tax_Data::HALF_FIVE) {
    $rounded_total = round($amount / 5) * 5;
} else {

round() requires a number, but $amount / 5 is a string. Either $amount needs to be converted to a number, or some type annotations are incorrect. This error also exists in app/Database/Migrations/20200202000000_taxamount.php (line 253) and potentially other migration files.