I can't download certain apps on my iPhone

Your iphone 6 plus can't be updated beyond 12.5.7.

Those apps require iOS 13.4 or higher.

If you want to use those apps, you will need to purchase a newer phone capable of running a higher operating system.

Try using the web versions.

What exact issue are you facing when you mean you can't download those apps? Does it provide any error message, ask for a password or similar?