def functions creation

[DeaD_EyE]

I think you think too complicated.

Given is max_volume, max_weight and density.

Then you calculate the volume: volume = max_weight / density
Then you return the minimum of max_volume and volume.

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def calculate_possible_volume(
    density,
    max_volume=67.5,
    max_weight=26199
    ):
    """
    Calculate the possible volume without
    hitting the max_weight limit.
    
    density := kg/m^3
    max_volume := m^3
    max_weight := kg

    return := m^3
    """
    volume = max_weight / density
    return min(max_volume, volume)

The test with your provided data:

Output:
>>> calculate_possible_volume(1800)
14.555
>>> calculate_possible_volume(380)
67.5

The if, elif, else and for-loop is misleading.
Solving the problem with simple math is simpler.

By the way, your test code is wrong.

def test_volume_per_container():
    """Test the volume_per_container function."""
 
    # Test for gravel at 1800 kg per cubic metre 
    assert volume_per_container(1800) == 14.555
     
    # Test for wood chips at 380 kg per cubic metre
    assert volume_per_container(380) == 67.5
     
    print ("tests passed") 

Should be:

from math import isclose


def test_volume_per_container():
    """Test the volume_per_container function."""
 
    # Test for gravel at 1800 kg per cubic metre 
    assert isclose(volume_per_container(1800), 14.555)
     
    # Test for wood chips at 380 kg per cubic metre
    assert isclose(volume_per_container(380), 67.5)
     
    print ("tests passed") 

Both testes, if you have the right function name, passing.
But the cause why you should always avoid float == float is 0.30000000000000004.com. Tell this your tutor. He should know this.