Missing kernel headers for RaspberryPi4 (32 bit OS)

[Dmitriy Mitchenkov]

Missing kernel headers for RaspberryPi4 (32 bit OS):
Config:
Hardware: RP4B, tp-link T2U plus;
OS: Clean Raspbian OS full (32bit);
Followed commands:

sudo apt-get update
sudo apt-get install raspberrypi-kernel-headers dkms -y
#sudo apt-get install bc mokutil build-essential libelf-dev linux-headers-`uname -r`
git clone -b v5.6.4.2 https://github.com/aircrack-ng/rtl8812au.git
cd rtl*
sed -i 's/CONFIG_PLATFORM_I386_PC = y/CONFIG_PLATFORM_I386_PC = n/g' Makefile
sed -i 's/CONFIG_PLATFORM_ARM_RPI = n/CONFIG_PLATFORM_ARM_RPI = y/g' Makefile
export ARCH=arm
sed -i 's/^MAKE="/MAKE="ARCH=arm\ /' dkms.conf
sudo make dkms_install

also I tried:

sed -i 's/CONFIG_PLATFORM_ARM_RPI = y/CONFIG_PLATFORM_I386_PC = n/g' Makefile
sed -i 's/CONFIG_PLATFORM_ARM64_RPI = n/CONFIG_PLATFORM_ARM64_RPI = y/g' Makefile
export ARCH=arm64
sed -i 's/^MAKE="/MAKE="ARCH=arm64\ /' dkms.conf
sudo make dkms_install

Error itself:

mkdir: created directory '/usr/src/8812au-5.6.4.2_35491.20191025'
cp -r * /usr/src/8812au-5.6.4.2_35491.20191025
dkms add -m 8812au -v 5.6.4.2_35491.20191025

Creating symlink /var/lib/dkms/8812au/5.6.4.2_35491.20191025/source ->
                 /usr/src/8812au-5.6.4.2_35491.20191025

DKMS: add completed.
dkms build -m 8812au -v 5.6.4.2_35491.20191025
Error! echo
Your kernel headers for kernel 6.1.21-v8+ cannot be found at
/lib/modules/6.1.21-v8+/build or /lib/modules/6.1.21-v8+/source.
You can use the --kernelsourcedir option to tell DKMS where it's located.
make: *** [Makefile:1786: dkms_install] Error 1

Additional info:
lsusb:

Bus 001 Device 003: ID 2357:0120 TP-Link Archer T2U PLUS [RTL8821AU]

ls -la /lib/modules:

total 24
drwxr-xr-x   6 root root 4096 May  3 03:06 .
drwxr-xr-x 107 root root 4096 Aug 28 00:16 ..
drwxr-xr-x   3 root root 4096 Aug 28 00:17 6.1.21+
drwxr-xr-x   3 root root 4096 Aug 28 00:17 6.1.21-v7+
drwxr-xr-x   3 root root 4096 Aug 28 00:17 6.1.21-v7l+
drwxr-xr-x   3 root root 4096 May  3 03:06 6.1.21-v8+

ls -la /lib/modules/6.1.21-v7l+:

total 2616
drwxr-xr-x  3 root root   4096 Aug 28 00:17 .
drwxr-xr-x  6 root root   4096 May  3 03:06 ..
lrwxrwxrwx  1 root root     34 Apr  5 17:01 build -> /usr/src/linux-headers-6.1.21-v7l+
drwxr-xr-x 11 root root   4096 May  3 03:06 kernel
-rw-r--r--  1 root root 617097 Apr  5 14:32 modules.alias
etc.

ls -la /lib/modules/6.1.21-v8+:

total 2652
drwxr-xr-x  3 root root   4096 May  3 03:06 .
drwxr-xr-x  6 root root   4096 May  3 03:06 ..
drwxr-xr-x 11 root root   4096 May  3 03:06 kernel
-rw-r--r--  1 root root 617502 Apr  5 14:32 modules.alias
etc.

ls -la /usr/src:

total 24
drwxr-xr-x  6 root root 4096 Aug 28 00:21 .
drwxr-xr-x 11 root root 4096 May  3 03:04 ..
drwxr-xr-x 25 root root 4096 Aug 28 00:17 linux-headers-6.1.21+
drwxr-xr-x 25 root root 4096 Aug 28 00:17 linux-headers-6.1.21-v7+
drwxr-xr-x 25 root root 4096 Aug 28 00:17 linux-headers-6.1.21-v7l+
drwxr-xr-x  3 root root 4096 May  3 03:14 sense-hat

Temporary solutiuon:
Just use (reinstall OS to) Raspbian OS 64bit, that have /usr/src/linux-headers-6.1.21-v8+ folder and run following commands:

#sudo apt-get update
#sudo apt-get bc mokutil build-essential libelf-dev linux-headers-`uname -r`
sudo apt-get install raspberrypi-kernel-headers dkms -y
git clone -b v5.6.4.2 https://github.com/aircrack-ng/rtl8812au.git
cd rtl*
sed -i 's/CONFIG_PLATFORM_I386_PC = y/CONFIG_PLATFORM_I386_PC = n/g' Makefile
sed -i 's/CONFIG_PLATFORM_ARM64_RPI = n/CONFIG_PLATFORM_ARM64_RPI = y/g' Makefile
export ARCH=arm64
sed -i 's/^MAKE="/MAKE="ARCH=arm64\ /' dkms.conf
sudo make dkms_install
cd ~

After running this commands you need to reattach the dongle (unplud and plug in again) or reboot your raspberry.

[Dmitriy Mitchenkov]

UPD:
Add "arm_64bit=0" to config.txt