An external camera may be advised; however which ones may work
with El Capitan may be sparse or few at this late a date. Specs change
and so that could create a problem. ~ Or you could be lucky!
In order to get correct one from online vendor could be minor issue.
To avoid endless exchanges or shipping issues. A good apple-centric
vendor may have something adequate.
I've a Mid-2012 MacBook Pro, with Mojave 10.14.6 installed, does OK.
Has 16GB RAM and 240GB blade SSD; but don't use it much though.
My Late 2012 Mac mini (2.3GHz 4-core server) can use older FireWire
Apple iSight; that sits on top of Apple Cinema 20-inch Display & has
built-in microphone. [Mini w/dual HDDs & 3 macOS, in 4 partitions.]
Troubleshooting the built-in camera may just be a matter of doing
a reset; perhaps SMC (system management controller) &/or NVRAM.
• How to reset the SMC of your Mac - Apple Support
https://support.apple.com/en-us/HT201295
• Reset NVRAM or PRAM on your Mac - Apple Support
https://support.apple.com/en-us/HT204063
Take care & good luck!🌞🌿⛵️🎣