Please see below a run-down of Focusrite product compatibility with Apple OS X 10.11 El Capitan as it currently stands:
Clarett 2Pre, 4Pre, 8Pre & 8Pre X
Works with El Capitan? YES - however there is a known issue with 5k Retina iMacs (model 17,1), please see this article
Clarett USB Range (2Pre USB, 4Pre USB and 8Pre USB)
Works with El Capitan? YES
Saffire firewire range (Pro 14, Pro 24, Pro 24 DSP, Pro 26, Pro 40, LS 56)
Works with El Capitan? YES
Scarlett 3rd Gen Range (Solo, 2i2, 4i4, 8i6, 18i8 and 18i20)
Works with El Capitan? Not officially tested or supported, but should work. You will need to install Focusrite Control 3.4.6 instead of the latest version.
Scarlett 2nd Gen Solo, 2i2 and 2i4
Works with El Capitan? YES
Scarlett 2nd Gen 6i6, 18i8 and 18i20
Works with El Capitan? YES
Scarlett 1st Gen Solo, 2i2 and 2i4
Works with El Capitan? YES
Scarlett 1st Gen 6i6, 18i8 and 18i20
Works with El Capitan? YES
Scarlett 1st Gen 8i6 and 18i6
Works with Catalina? YES
Saffire 6 USB 2.0
Works with El Capitan? YES
Points of note: If you are using the USB 2.0 version of the Saffire 6, no driver is required. Simply plug and play. If you are unsure whether your unit is the USB 1.1 or 2.0 version please check the serial number shown on the bottom of the device. If the serial number starts 'S2' it's a Saffire 6 USB 2.0, if it starts 'SU', it's a Saffire 6 USB 1.1.
Saffire 6 USB 1.1
Works with El Capitan? YES
Forte
Works with El Capitan? YES
iTrack Solo
Works with El Capitan? YES
VRM Box
Works with El Capitan? NO. The VRM Box is no longer supported. We highly recommend you do not update to El Capitan if you plan to continue using VRM box.
RedNet
Works with El Capitan? YES
Red 2 & Red 3 plug-in suite
Works with El Capitan? YES
Midnight plug-in suite
Works with El Capitan? YES
Scarlett plug-in suite
Works with El Capitan? YES
Points of note: If you have problems locating your license file please switch to Icon View. The folder may appear empty if you are in List View to Column View.
If your product is not listed, it is not officially supported with OS X El Capitan, you can find full details on our support policy here.