gcc version 6.4.0 20180424 (Ubuntu 6.4.0-17ubuntu1)
*** /proc/version
*** ls: -r–r–r-- 1 root root 0 2021-03-26 11:05:06.306296949 +0300 /proc/version
Linux version 5.8.0-48-generic (buildd@lgw01-amd64-008) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #54~20.04.1-Ubuntu SMP Sat Mar 20 13:40:25 UTC 2021
As suspected a compiler mismatch. Your kernel was built with gcc v.9.3. But you try to compile the kernel module with 6.4.
Please switch cc and gcc to 9.3 using update-alternatives.