I know I'm a few months late, but adding the line
annotationProcessor 'org.spongepowered:mixin:0.8.5:processor'
to your dependencies should help fix this.
so it should look like this:
dependencies {
minecraft 'net.minecraftforge:forge:1.18.2-40.2.0'
annotationProcessor 'org.spongepowered:mixin:0.8.5:processor'
}