This does not work because this is not how you merge two arrays:
'post__not_in' => get_upsell_ids(), [get_the_ID()]
Instead you need to take those 2 arrays and combine them using array_merge, a standard PHP function:
https://www.php.net/manual/en/function.array-merge.php
Note that __not_in type parameters have a very high performance cost and do not scale, this query will be very slow and require caching.