Simply use:
call CRC32Checksum
In FASM when you bracket something in square brackets it means indirect addressing.
"call [CRC32Checksum]" means "call the procedure which address is located at CRC32Checksum variable. In practice, you will get compiler error "operand size not specified" because the label CRC32Checksum is code label and has no size assigned. (You can overwrite this by using "call dword [CRC32Checksum]" but here it is meaningless of course).
Note that you call the imported function "getchar" by indirect call. This is because the imported functions are actually dword variables that contains the address of the imported functions.