Make old snow Leopard MacBook safer

I have an old MacBook running Snow Leopard 10.6.8. Specs are 2.4 GHz Intel Core Duo, 2GB RAM. I bought it for my gf (now wife) around 2008 and we have not used it in the last couple of years.

The OS and the software as a whole are rather outdated and I am most concerned about safety issues. So I would like to upgrade it and use it for some specific tasks.

Problem is: the link from the App Store to upgrade to El Capitan fails because I cannot upgrade but it doesn't tell me the reason. Space on disk is plenty (just above 36 GB).

What can I do? It would be a shame to be unable to use it. Is there some other upgrade path? Is there some other solution to make this box as usable (safely!) as possible?

Do you have another Mac that you can use to download the El Capitan installer (you should be able to access the link from any Mac running High Sierra or below). Then you can just install it on the old MacBook from there, or from an external drive. Sometimes you have to find ways to work around the roadblocks Apple puts up. But you should have no problem upgrading that machine to at least El Capitan (my 2008 MacBook unibody is currently running Mojave, thanks to a patched version of macOS, but El Cap is the last officially support version for those machines).

To answer your security question, according to Apple’s security updates page: https://support.apple.com/en-us/HT201222 they are still issuing security updates to El Capitan and to the version of Safari running on it, so you should be in good shape as long as you can get your Mac upgraded.

p.s. I’d recommend maxing out the RAM and swapping out the slow spinning HDD for an SSD to breathe new life into the machine. Whatever you choose to run will work a whole lot better with an SSD especially.