Unable to Generate .sof File in Quartus Prime Standard Edition 15.1 – 30‑Day Evaluation Mode

No — the 30‑day evaluation mode of Quartus Prime Standard Edition does not support generating programming files, including .sof.
https://www.macnica.co.jp/en/business/semiconductor/support/faqs/intel/118773/

Since the evaluation itself does not permit .sof generation, there are only two valid options:
-Option A — Use Quartus Prime Lite Edition (Free):
Quartus Lite does allow .sof generation and does not require a license, provided your design does not use licensed IP. This is the recommended method for evaluation requiring programming. 
-Option B — Use a Licensed (Non‑Evaluation) Quartus Standard Edition:
A valid Standard (or Pro) Edition license will allow full .sof generation. The 30‑day evaluation mode alone will not.

Regards,
Richard Tan