MASM: how to resolve error "immediate operand not allowed"

My assembly program has following structure:

BPB_FAT16   STRUCT                     ; Base BPB_FAT16. Size=0x19=25.
BytesPerSector      DW  0x0200 ; Size of HW sector, usualy 512.
SectorsPerCluster   DB  0x01   ; Valid values 1,2,4,8,16,32,64,128.
ReservedSectors     DW  0x0001 ; NrOfSectors preceding FAT.
NumberOfFats        DB  0x02   ;
RootEntries         DW  0x00E0 ; Max number of YWORD entries in the 
                               ;root dir.
SmallSectors        DW  0x0B40 ; See also .LargeSectors.
MediaDescriptor     DB  0xF0   ; 0xF0 floppy disk, 0xF8 hard disk.
SectorsPerFat       DW  0x0009 ;
SectorsPerTrack     DW  0x0012 ;
NumberOfHeads       DW  0x0002 ;
HiddenSectors       DW 0x00000000
LargeSectors        DW 0x00000000
                               ; Extended BPB_FAT16. Size=0x1A=26.
PhysicalDriveNumber DB 0x00   ; 0x00 floppy disk, 0x80 hard disk.
Reserved            DB 0x00   ;
ExtBootSignature    DB 0x29   ; Or 0x28.
VolumeSerialNumber  DW 1212121 ; Randomly generated number.
VolumeLabel         DB "NO NAME    " ; Space-padded to size=11.
FileSystemType      DB "FAT12   "    ; Space-padded to size=8.
BPB_FAT16 ENDS  ;   Total BPB_FAT16. Size=0x33=51.

My program is showing two errors for only one line of code given below:

Bpb   BPB_FAT16{}

and those two errors are:

error1: missing operator in expression
error2: initializer magnitude too large for specified size

What should I do? Please guide me.

The error message missing operator in expression is likely occurring on every line where you use 0x prefix for hexadecimal values. MASM doesn't support that and it use the h suffix to denote hexadecimal values.

The error initializer magnitude too large for specified size is because the value 1212121 can't fit in the DW (16-bit WORD) you specified for VolumeSerialNumber. VolumeSerialNumber in a BPB needs to be a DD (32-bit DWORD).

Although it didn't generate an error, the fields HiddenSectors and LargeSectors in a BPB are a 32-bit DWORD so they should be using the type DD instead of DW.

Your structure could have been defined as:

BPB_FAT16   STRUCT                   ; Base BPB_FAT16. Size=0x19=25.
BytesPerSector      DW  200h         ; Size of HW sector, usualy 512.
SectorsPerCluster   DB  1h           ; Valid values 1,2,4,8,16,32,64,128.
ReservedSectors     DW  1h           ; NrOfSectors preceding FAT.
NumberOfFats        DB  2h           ;
RootEntries         DW  0E0h         ; Max number of YWORD entries in the
                                     ; root dir.
SmallSectors        DW  0B40h        ; See also .LargeSectors.
MediaDescriptor     DB  0F0h         ; 0xF0 floppy disk, 0xF8 hard disk.
SectorsPerFat       DW  9h           ;
SectorsPerTrack     DW  12h          ;
NumberOfHeads       DW  2h           ;
HiddenSectors       DD  0h
LargeSectors        DD  0h
                                     ; Extended BPB_FAT16. Size=0x1A=26.
PhysicalDriveNumber DB  0h           ; 0x00 floppy disk, 0x80 hard disk.
Reserved            DB  0h           ;
ExtBootSignature    DB  29h          ; Or 0x28.
VolumeSerialNumber  DD 1212121       ; Randomly generated number.
VolumeLabel         DB "NO NAME    " ; Space-padded to size=11.
FileSystemType      DB "FAT12   "    ; Space-padded to size=8.
BPB_FAT16 ENDS  ;   Total BPB_FAT16. Size=0x33=51.

With these changes your structure is now exactly 51 bytes which is what you'd expect for a FAT16 DOS 4.0 EBPB.