Makefile:775: arch/armv7l/Makefile: No such file or directory

[savjanivv]

Hi all,
Apologies is this is a very noob question. When trying to build this on RPI 2b with RTL8851BU adapter, I'm getting the following issue:

$make
make ARCH=armv7l CROSS_COMPILE= -C /lib/modules/6.1.21-v7+/build M=/home/pi/rtl8831 modules
make[1]: Entering directory '/usr/src/linux-headers-6.1.21-v7+'
Makefile:775: arch/armv7l/Makefile: No such file or directory
make[1]: *** No rule to make target 'arch/armv7l/Makefile'. Stop.
make[1]: Leaving directory '/usr/src/linux-headers-6.1.21-v7+'
make: *** [Makefile:827: modules] Error 2

When I have created the directory and added the Makefile from the arm directory, the error becomes:

make ARCH=armv7l CROSS_COMPILE= -C /lib/modules/6.1.21-v7+/build M=/home/pi/rtl8831 modules
make[1]: Entering directory '/usr/src/linux-headers-6.1.21-v7+'
CC [M] /home/pi/rtl8831/platform/platform_ops.o
/home/pi/rtl8831/platform/platform_ops.c:15:10: fatal error: drv_types.h: No such file or directory
15 | #include <drv_types.h>
| ^~~~~~~~~~~~~
compilation terminated.
make[2]: *** [scripts/Makefile.build:250: /home/pi/rtl8831/platform/platform_ops.o] Error 1
make[1]: *** [Makefile:2012: /home/pi/rtl8831] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-6.1.21-v7+'
make: *** [Makefile:827: modules] Error 2

Any help for armv7l users - seems like it's been an issue for other devs who have managed to implement something for non 64bit arm processors (diederikdehaas/rtl8812AU#103)?
As a reasonably inexperienced user, I'm happy to try anything/provide more specific info if needed.

[savjanivv]

UPDATE:
I re-install Raspian 12 (bookworm).
Now the error is:

make ARCH=armv7l CROSS_COMPILE= -C /lib/modules/6.6.51+rpt-rpi-v7/build M=/home/vishal/RTL8851bu modules
make[1]: Entering directory '/usr/src/linux-headers-6.6.51+rpt-rpi-v7'
CC [M] /home/vishal/RTL8851bu/platform/platform_ops.o
In file included from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/bitmap.h:11,
from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/cpumask.h:12,
from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/mm_types_task.h:14,
from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/mm_types.h:5,
from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/buildid.h:5,
from /usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/module.h:14,
from /home/vishal/RTL8851bu/include/basic_types.h:38,
from /home/vishal/RTL8851bu/include/drv_types.h:24,
from /home/vishal/RTL8851bu/platform/platform_ops.c:15:
/usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/string.h:72:9: error: conflicting types for ‘strscpy’; have ‘ssize_t(char *, const char *, size_t)’ {aka ‘int(char *, const char *, unsigned int)’}
72 | ssize_t strscpy(char *, const char *, size_t);
| ^~~~~~~
In file included from /home/vishal/RTL8851bu/include/drv_types.h:23:
/home/vishal/RTL8851bu/include/drv_conf.h:715:26: note: previous declaration of ‘strscpy’ with type ‘size_t(char *, const char *, size_t)’ {aka ‘unsigned int(char *, const char *, unsigned int)’}
715 | #define strlcpy(a, b, c) strscpy(a, b, c)
| ^~~~~~~
/usr/src/linux-headers-6.6.51+rpt-common-rpi/include/linux/string.h:69:8: note: in expansion of macro ‘strlcpy’
69 | size_t strlcpy(char *, const char *, size_t);
| ^~~~~~~
make[3]: *** [/usr/src/linux-headers-6.6.51+rpt-common-rpi/scripts/Makefile.build:248: /home/vishal/RTL8851bu/platform/platform_ops.o] Error 1
make[2]: *** [/usr/src/linux-headers-6.6.51+rpt-common-rpi/Makefile:1946: /home/vishal/RTL8851bu] Error 2
make[1]: *** [/usr/src/linux-headers-6.6.51+rpt-common-rpi/Makefile:246: __sub-make] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-6.6.51+rpt-rpi-v7'
make: *** [Makefile:825: modules] Error 2