Set Line Item TaxCode via REST API
I can confirm that, whilst I get an error for an invalid value, I cannot get the value to set correctly.
There have been a couple of recent issues that suggest there might be something not working correctly when it comes to selections on the Item sublist:
After some googling, I came across this post https://community.oracle.com/netsuite/discussion/4473977/setting-tax-code-for-sales-order-item-through-rest-api, which suggests that assigning line-level taxes don’t work with REST API unless you have the SuiteTax feature enabled. Can you confirm you have this enabled for your NS account?