Without the \"$", it will compile.
You only specify the length to match. There is no optional terminating character.

But, I think maybe there's some confusion about what WAITSTR does ...
It waits until it receives a string of text that matches the contents of the specified array.

If you want it to wait for "all done", you first have to load the array with the text.

ARRAYWRITE Ser_data,["all done"]
HSERIN [waitstr Ser_data\8]

If you want the "$" in there too, you just add it to the string ...

ARRAYWRITE Ser_data,["all done$"]
HSERIN [waitstr Ser_data\9]

If it never receives "all done$", it will never exit the command unless you add a timeout.