It’s because Fiverr still haven’t made it work the way the level help page says it should.
The level help page talks about the max active for a level, but the Fiverr system takes inactive gigs into account then deciding whether to allow a new gig to be created. At least level 0.
You could create a ticket and ask if they could make it work the way it should ie. that the gig creation shouldn’t stop you creating a new gig if you have less than the maximum allowed active gigs for your level. It could be classed as a bug the way it currently works.