The message
from James Fidell contains these words:

Janet Galpin wrote:

I think celery is more fussy than celeriac. Did you have them in enough
warmth?

I'd hope so. I had several batches, some in a sunny window and others
in the propagator. There's been no sign of life from either.

James

Joy Larkcom specifies 50-59F so I suppose the other possibility is that
both sites were too warm. However, I germinated mine in the airing
cupboard which gets very warm so I would doubt that is the reason.
Next step would be to suspect the viability of the seed. Was it well
within its sell-by date? Could it have been from a garden centre where
it might have been kept in conditions which were too warm?
You could try contacting the seed company. Some are very good at
replacing seed in situations where you've clearly tried it in a range of
conditions.

Janet G