This is a common problem, I've fixed it several times in my 55 years. You
may have to completely clean out the freezer of food and really stick your
head back in there. If it's clogged with "stuff" it may have water or ice
covering it.
Look in the bottom reefer section and see if there isn't a bulge slanting
and coming down from the top towards the rear. That will give you an idea of
where to look.
Is there a drain pan underneath the unit with a tube going to it? If ithe
pan's bone dry and dusty, you've probably got a clog up top.

Gerry

I have not found a drain tube in the freezer. I removed a plastic cover
from the floor of the freezer. Under it was a slab of 1/4"-thick ice,
which I removed. then I removed the back cover of the freezer but still
did not find the vent pipe.

Any other suggestions?

Thanks,
Arnold