Tile Error in Google Earth Engine?

Not every image in the images you've selected has the properties GAIN_COEFFICIENT_B04 and so on. For example, ASTER/AST_L1T_003/20100709070749 has only

GAIN_COEFFICIENT_B01: 0.676
GAIN_COEFFICIENT_B02: 0.708
GAIN_COEFFICIENT_B10: 0.006822
GAIN_COEFFICIENT_B11: 0.00678
GAIN_COEFFICIENT_B12: 0.00659
GAIN_COEFFICIENT_B13: 0.005693
GAIN_COEFFICIENT_B14: 0.005225
GAIN_COEFFICIENT_B3N: 0.862

Thus, .get('GAIN_COEFFICIENT_B04') returns null, and your image calculation fails because null is not a valid value for a constant image.

You can add a filter on the image collection to ensure those properties are present:

.filter(ee.Filter.notNull([
  'GAIN_COEFFICIENT_B04',
  'GAIN_COEFFICIENT_B05',
  'GAIN_COEFFICIENT_B06',
  'GAIN_COEFFICIENT_B07',
  'GAIN_COEFFICIENT_B08',
  'GAIN_COEFFICIENT_B09',
]))

---

I noticed some other improvements that could be made to your code.

  var gain4 = ee.Image(ee.Number(img.select('B04').get('GAIN_COEFFICIENT_B04')));

Here, the .select('B04') does nothing (since you're getting a property, and properties are independent of bands). Also, there are variants of .get() that eliminate needing to write ee.Number:

  var gain4 = ee.Image(img.getNumber('GAIN_COEFFICIENT_B04'));

But this can be even simpler: it isn't necessary to write ee.Image(constant) in many cases; you can write a number and it will be converted to an image.

  var b4 = img.select('B04').multiply(img.getNumber('GAIN_COEFFICIENT_B04')).clip(table);

Finally, it is unnecessarily inefficient to .clip(table) the individual images rather than the result, and when you are clipping to a table/collection you should use the more efficient .clipToCollection(table) (though this doesn't matter since you only have one feature).

.map(function(img){
  ...
}).median().clipToCollection(table);

Putting it all together:

var ast = ee.ImageCollection("ASTER/AST_L1T_003")
  .filterDate('2000-01-01','2020-01-01')
  .filterBounds(table)
  .filter(ee.Filter.calendarRange(7,9,'month'))
  .select('B0[4-9]')
  .filter(ee.Filter.notNull([
    'GAIN_COEFFICIENT_B04',
    'GAIN_COEFFICIENT_B05',
    'GAIN_COEFFICIENT_B06',
    'GAIN_COEFFICIENT_B07',
    'GAIN_COEFFICIENT_B08',
    'GAIN_COEFFICIENT_B09',
  ]))
  .map(function(img){
    return ee.Image.cat([
      img.select('B04').multiply(img.getNumber('GAIN_COEFFICIENT_B04')),
      img.select('B05').multiply(img.getNumber('GAIN_COEFFICIENT_B05')),
      img.select('B06').multiply(img.getNumber('GAIN_COEFFICIENT_B06')),
      img.select('B07').multiply(img.getNumber('GAIN_COEFFICIENT_B07')),
      img.select('B08').multiply(img.getNumber('GAIN_COEFFICIENT_B08')),
      img.select('B09').multiply(img.getNumber('GAIN_COEFFICIENT_B09')),
    ]);
  })
  .median()
  .clipToCollection(table);

https://code.earthengine.google.com/3ba97a7c2009def6043c614c5aeae4f7