Thank you so much for your explanation.

I want to explain the situation more to avoid any confusion.

Country Variable I have 50 entries

Outcome (A) Continuous Variable I have ( 20 columns for each country)

Predictor (B) Continuous Variable I have ( 20 columns for each country)

Predictor (C) Continuous Variable I have ( 20 columns for each country)

Predictor (D) Continuous Variable I have ( 20 columns for each country)

So in total, I have 1000 entries for the outcome/predictors

The photo below explains how my data is collected. I did not include the 20 years for each.

Can you please explain how I should adjust, given that each country has 20 values that follow particular growth? And to account for that, we have 1000 different values for each variable.

How do we describe the adjusted model then?