Is there a sales tax on a replacement skylight. Or is is it a capital improvement?

You are asking about 2 different issues.

Yes, there will be sales tax for the part.

As to how you claim it on your taxes, that depends on whether you live at the property or not - and whether you are replacing an old one or adding one where there was not one before.

Based on what you've told us, here's what I would do (this is just my opinion):

If it is for a rental property, I'd write it off as maintenance.

If it is for my home where I live, I'd call it a capital improvement.

Clarify a bit more and we can probably give you a clearer answer.

Are you asking whether a contractor who replaces your skylight should charge you sales tax on either the skylight or his services? That depends on the sales tax rules for your jurisdiction, but in many places the answer is yes.