..sulayman100..
Sometimes your Mac may need an upgrade to nearer the target system;
if the Support site can't get a downloadable installer from there; & Balks..
..You may need to have at least High Sierra installed before a higher OS
could be downloaded. ~ From there you may get Mojave; & Catalina..
Where this happened at least once (prior to recent issues with downloads
from the upgrades page, where they've failed more often than usual) was
just Before my Mac mini (a dual HDD server) had seen install of High Sierra,
up from El Capitan. Both High Sierra & Mojave downloaded installers were
invisible; a year went by; and I re-downloaded a newer High Sierra 10.13.6.
With that one, installed, the Mac noticed the other untouched downloads
and then started to download a few newer patches for those too. Those are
on different internal drives; were only seen after 'a key' Mac system was run.
I'd tried to download today from the Mac app store and it failed; with Safari.
Even tried to re-download second copy, that should have been seen. No dice.
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
The main portal link seems to work OK, while content that should be there, is
not always accessible to next applicable macOS, that fits the download puzzle.
While you may have better luck with timing and internet access than me; this
other matter relies on the Installers you finally get, are in proper cue. Only then
they may get a newer supported system download installer from Apple servers.
There are hurdles to pole-vault over in this 'mini-olympian' struggle to upgrade.
Hopefully your luck and timing allow a less arduous interval, with less waiting.
Good luck & happy computing! 🤖🙀🤡🐶