2001 Cadillac Catera. Check engine  is flashing.
Replaced Water pump. Fuel filter.  Valve cover gaskets coils. New
plugs oil change.
Question = I recently replaced valve cover gaskets and when I
started the car it drove no faster than 10 mph. Looking for a solution
to fix this problem.
Answer: There should be no correlation with doing valve cover
gaskets and then not being able to go over 10 mph. That is, unless
some went wrong during the repair. If a gasket isn't seated properly,
or something was left loose or off, there could be a vacuum leak and
that would affect driving. Otherwise, I'd look elsewhere for the
problem. Possibly the PCV line was left off or missing? Check any and
all o-ring seals are in place, too. You mention in your info that the
check engine light is flashing which usually means that a misfire on
one or more cylinders is happening. If it's not being caused by a
vacuum leak, you have either an ignition problem, fuel problem or
low/no compression due to a valvetrain problem.