Any suggestions as to where I might be going wrong?

I’ve seen this. Try uploading as a 2.0 script and then change the version after. I think there’s an issue with creating a 2.1 client script from scratch.

I think I experienced some sort of similar caching issue a while back as well - if I recall, making any kind of inline change in the NetSuite embedded code editor also seemed to resolve potentially

Yeah. That worked. Thanks. I think netsuite should have to pay for all the time I spend on these stupid bugs

I just had a similar issue and same error message so thought I'd comment here for anybody else that searches for this later. I declared some variables using "let" but the API version was set to 2.x. NetSuite doesn't like that so I had to either change it to "var", or force the API version of the client script to be 2.1. Either way resolved the issue.