Question about Determinant in solve block

Can someone tell me what is wrong with the setup of the solve block?

I have trouble using the determinant operator is solve block.

When I set a determinant of a matrix say, |M| = 0, it returns an error saying the RHS of the determinant must be a vector of scalars

Even when I use determinant operator to return the norms of a vector, it is still an error

I tried to simply define a new variable M1:= |M| and M1=0

so I do not have to actually use the determinant operator in the solve block, but still no luck

do not understand what is wrong

Thanks