Explain two economic issues related to population growth.

Population growth can result in many economic issues, however, we will focus on these two: economic dependency and unemployment.
Economic dependency
a) When a country experiences population growth, this can result in a larger labour force in the future. However, the initial increase in population can put a strain on the country's resources. This because between the ages of 0 to 16 or 18 (depending on the labour laws of a country) these children are dependent on their parents, other caregivers, or the state.
b) The financial strain on the resources of a country can extend to its educational, health, housing, and infrastructural facilities. Therefore, government and the persons in large families will have to allocate scarce resources to providing and obtaining access to more of these facilities.
Unemployment
a)  In the future, when the population is grown and trained in their desired professions; they may be unable to access jobs because the aging population (their parents and other adults) are still holding these positions. This younger section of the labour force may subsequently become discouraged and either stop looking for work or migrate in search of better employment opportunities (brain drain).
https://blogs.worldbank.org/africacan/can-rapid-population-growth-be-good-for-economic-development

One issue of population growth is scarcity.  A populous region demands a lot of food, fuel, and housing.  This drives up the costs of these three things and can make it so that only a select few can afford these necessities.  Many large urban areas have high rents and it is expensive to buy food there.  Those who cannot afford this live in worse conditions than they would in a less populated rural area.  Nations with high population densities often have low standards of living for a large number of their populations.  
 
Another issue is waste.  People are consumers of food and fuel and this leads to pollution unless it is properly managed.  Populous areas such as London and New York City used to dump sewage into rivers during the nineteenth century--this led to disease outbreaks.  Air quality in Mexico City and Los Angeles, two very populous areas, suffers during rush hour.  Of course, as the Earth's population increases, there is also more discussion about how to control carbon emissions in order to prevent global warming.