Ok there are a few problems here.

1. You have random slopes for gini but you do not specify it as a fixed effect. It's your main exposure so it should be a fixed effect.

2. The model has a singular fit. By removing gini as a random slope this may fix that although it's likely that AbroadIncome is causing that.

I would suggest starting with the following model:

Migration ~ Rich_Poor_Indicator +  Gini +

AbroadIncome + (1 | Country.Name)

If this is supported by the data then you can consider adding random slopes.

Ok there are a few problems here.

1. You have random slopes for gini but you do not specify it as a fixed effect. It's your main exposure so it should be a fixed effect.

2. The model has a singular fit. By removing gini as a random slope this may fix that although it's likely that AbroadIncome is causing that.

I would suggest starting with the following model:

Migration ~ Rich_Poor_Indicator +  Gini + AbroadIncome + (1 | Country.Name)

If this is supported by the data then you can consider adding random slopes.