Variable issue

You should define A(t) as a function like this

The way you defined it the function would only return 0 or 1 (false or true) because it was a boolean expression.

Now you can evaluate it at any position t

Unfortunately the symbolic solver is simply not capable enough to solve your equation

So you should use the numeric methods of Prime, either a solve block with "find" or the "root" function.

As your equation has multiple solutions as Cornel already stated, you get different solutions for different guess values

You may also use the root function without a guess value if you provide a range for the t-values

Here you see these three solution in the plots

We cannot distinguish the first two so we zoom in

And in case you are actually looking for the position(s) of the maximum, you may use the derivative in the root function

or at a different position and this time using the root function with a guess