Soil mixes sold here aren't always so good. Did you recently buy some soil and use that to fill the bed? Wouldn't be the first time I've seen newly planted stock go yellow when stuck in locally sold commercial planting mix.

Buy a 4 lb box of organic fertilizer at your local nursery. Use half the box, water well.

Thank you.
The soil is part of a big load of topsoil from an area supplier that I use for all my flower beds. Last year I put in 2 raised beds. The other plants (including a yukon gold potato in the other) in both are doing fine but not this zuchini. Would nitrogen help?

Depend on what, specifically, is wrong with the one plant.