As your mac is running a very old operating system, Mac OS X Yosemite 10.10.5 with a very old version
of Safari then you will find it no longer connects to many websites as it does not have the modern
security protocols to connect.
You would need to upgrade the operating system of your mac so you can connect to these websites.
The good news is that your 2015 MacBook Pro can upgrade to more modern macOS's including the
latest macOS Big Sur.,
Although Apple say you can upgrade directly to macOS Big Sur from Yosemite it is rather a large leap.
I would recommend that you upgrade to macOS High Sierra 10.13 first as this will prepare your mac for
the new APFS file system that was introduced in High Sierra. Once you have installed High Sierra you can
then consider if you want to upgrade to macOS Mojave 10.14, macOS Catalina 10.15 or macOS Big Sur 11.4.
For High Sierra, Mojave, Catalina,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on the OS you want to install.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you want to install Big Sur, open Safari and click on the link below.
How to upgrade to macOS Big Sur - Apple Support
If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.