default or selected kernelversion does not have config value CONFIG_COSA.
Result is shown for kernelversion 6.9.9

COSA/SRP sync serial boards support

modulename: cosa.ko

configname: CONFIG_COSA

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>Wan interfaces support
└─>COSA/SRP sync serial boards support
In linux kernel since version 2.6.12  
Driver for COSA and SRP synchronous serial boards.

These boards allow to connect synchronous serial devices (for example
base-band modems, or any other device with the X.21, V.24, V.35 or
V.36 interface) to your Linux box. The cards can work as the
character device, synchronous PPP network device, or the Cisco HDLC
network device.

You will need user-space utilities COSA or SRP boards for downloading
the firmware to the cards and to set them up. Look at the
<http://www.fi.muni.cz/~kas/cosa/> for more information. You can also
read the comment at the top of the <file:drivers/net/wan/cosa.c> for
details about the cards and the driver itself.

The driver will be compiled as a module: the
module will be called cosa.

source code:
drivers/net/wan/cosa.c