Work contract

If you are registered then yes you have to

If you are gst registered then you have to issue invoices

If you are registered under GST and the supply is a taxable supply, then you have to issue GST Tax Invoice in accordance with the Provision of Section 31 of the CGST Act 2017.

However if you are not a registered person, then there is no requirement of issuing tax invoice since you cannot collect GST from your customer being an unregistered person [ Section 32 of the CGST Act 2017 - Prohibition of Unauthorized Collection of Tax ].

Yes every reqular registered person in GST is required to raise Tax Invoice as per provision of Section 31 r/w rule 46 of CGST Act.