I can't seem to advertise for the Birthday WRC I get a popup message saying
Oops!

Your Garden does not appear to be set up to attract the Birthday Wriggly Red Caterpillar.

Please re-check the requirements to attract this wildlife and you will be able to advertise here when you have the correct food/plants in your Garden.

If you are still unsure why your Garden setup is incorrect, there is a dedicated 'Garden Setup Help' section on our forum page where you can ask other players for assistance :)

The sparkling rainbow butterpillaflora changes all WRC to coloured caterpillars.

oh.. but it says there isnt any deterrents

They don't deter.  They change the colour.  If you want WRC you need to ditch the butterpillaflora, or let the butterfly be spotted.  You won't get the other colours then, just WRC (and birthday WRC on cake).