I came across your question in Close Votes Review.
From the information available there, it seemed you had answered your own question with the comment:
@mustaccio not sure why I missed it in the question, RDS doesn't allow you to grant
CREATE on a tablespace (apparently). – Philip Couling 18 hours ago
That is, it seemed the question was a trivial one due to an oversight. Hence "too localized" seemed the right closure reason ("this could be because your code has a typo, basic error, or is not relevant to most of our audience").
The other two comments also seemed not to have been addressed:
Doesn't the PostgreSQL log file tell you exactly what statement was executing when the error was thrown? – jjanes 23 hours ago
Doesn't the documentation page you link to state explicitly what privilege the function requires? – mustaccio 21 hours ago
On reflection, and with the greater tools available outside the limited Review Task view, I see you had edited your question - just not made it clear you had addressed the points raised in comments. The comments had not been flagged as No Longer Needed either. It was hard to see what had happened.
Now you say:
And as it happens I've found a working solution right before commenting so the question was in fact answerable all along.
Which is good to hear, and I'd be happy to reopen for that reason alone in most cases. Regardless, I see my initial view of the question was incorrect and will therefore retract my close 'vote'. The only way I can do that (as a moderator) is to reopen the question.
For future questions, it would be helpful to make it clear you have addressed comments and ideally flagged any that no longer serve their primary purpose of leading to improvements in the parent post. I have removed those comments now.