#DIV/0! error where there was none before

Carefully check through columns M and O... there must be a #DIV/0 error somewhere in those columns....especially where there might have been a match...

BTW: you don't need to repeat the sheet name for the top and bottom of each range.... this'll shorten the formula a bit:

=SUM(IF('NBA Line Watching'!M31:M9999="ATL",IF('NBA Line Watching'!O31:O9999="W-b",1,0)))+SUM(IF('NBA Line Watching'!M31:M9999="ATL",IF('NBA Line Watching'!O31:O9999="W-bi",1,0)))+SUM(IF('NBA Line Watching'!M31:M9999="ATL",IF('NBA Line Watching'!O31:O9999="W",1,0)))+SUM(IF('NBA Line Watching'!M31:M9999="ATL",IF('NBA Line Watching'!O31:O9999="W-i",1,0)))

or even shorter and better...you can use sumproduct and you don't need to confirm with CTRL+SHIFT+ENTER. Just ENTER will suffice:

=SUMPRODUCT(('NBA Line Watching'!M31:M9999="ATL")*(('NBA Line Watching'!O31:O9999="W-b")+('NBA Line Watching'!O31:O9999="W-bi")+('NBA Line Watching'!O31:O9999="W")+('NBA Line Watching'!O31:O9999="W-i")))

Still better and shorter and easier to add more conditions is to add an Isnumber(Match()) check....

=SUMPRODUCT(('NBA Line Watching'!M31:M9999="ATL")*ISNUMBER(MATCH('NBA Line Watching'!O31:O9999,{"W-b","W-bi","W","W-i"},0)))