First, remember that the operations are executed from right to left. For example side=`B & price <= ask is equivalent to side=(`B & (price <= ask)). So you are trying to do an AND between a boolean and a symbol which leads to a type error. You should use parentheses e.g. around side=`B if you want that to be executed first.
Second, the $ conditional takes a single value, not a vector. There is a vector conditional operation that works on vectors instead.
Third, you can take advantage of operators such as <= >= & being atomic. You shouldn't need to use any kind of conditional - in general the pattern $[x;1b;0b] (or the vector version ?[x;1b;0b]) can be replaced by just x, and the two branches that return true can be joined together with an or instead.
