Custom Column if then else error

In Power Query, the M language has a different syntax for "if" statements compared to DAX

try it like this:

if [FY23 Flag] > 0 then "Retained"
else if [FY23 Flag] = 0 then "lapsed"
else "NewFY24"

you can try this

= if  xxx and xxxx then xx else xxx

pls see the official doc below

https://learn.microsoft.com/en-us/powerquery-m/m-spec-conditionals?wt.mc_id=DP-MVP-5004616