From the linked documentation's Introductory section #1:

The library for HTTP communication (LHTTP) enables the data exchange of a SIMATIC S7-1500 PLC

The LHTTP library is for S7-1500 PLCs.

To do what you want means you need to replace the s7-1200 with an s7-1500 PLC.

AutoGen_5304903

Hello,

I'd like to download data to S7-1200 from the following address:

https://api.raporty.pse.pl/api/rce-pln

In order to establish https connection I downloaded to the PLC certificates from the website as described in section 2.3  of the manual:

https://cache.industry.siemens.com/dl/files/879/109763879/att_987991/v2/109763879_LHTTP_DOC_V10_en.pdf

I tried using LHTTP_Get function from LHTTP library, but it returns error with status code 8601 and subfunction status code 80E4 what means No valid certification authority found from TCON function.

When I try to download similar json from the http server working in the local network (without TLS) everythink works ok.

PLC 1211C with firmware 4.6.

Tia Portal V19.