There is no MicroSD card availability for the S21 series of devices.
Your screenshot and the website only show the S20 series having external storage. No mention of the S21.
Please post a screenshot of your S21 device storage with external storage being used.
Go to... Settings >Battery and device care > Storage
"All three phones in the S21 series — the base S21, S21+, and S21 Ultra — all lack microSD expansion."
Correct... and the dual/hybrid SIM works in the S8 to S20 series, perfectly.
However as any S21 owner will confirm, this feature is not available for the S21 series.
"It’s official: Samsung is ditching the microSD card on all three of its Galaxy S21 models."
Please post your screenshot of the S21 using external storage as requested previously.