Your docstrings say brawlstars.ValueError. However, this is misleading because it's not brawlstars.ValueError, because ValueError is built-in to python. Change brawlstars.ValueError to just ValueError

Traceback (most recent call last):
File "C:\Python39\lib\site-packages\brawlstars\client.py", line 66, in get_player
raise HTTPError(resp.status_code)
File "C:\Python39\lib\site-packages\brawlstars\errors.py", line 29, in init
self.error = 'An error occurred. Status: ' + code
TypeError: can only concatenate str (not "int") to str

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "D:\prog\PycharmProjects\untitled3\brawltracker.py", line 4, in
player = client.get_player(tag="Q8P2ULP")
File "C:\Python39\lib\site-packages\brawlstars\client.py", line 72, in get_player
raise Timeout()
brawlstars.errors.Timeout

Process finished with exit code 1.

code must be str?

client.py

• from .errors import * - don't use wildcard imports
• self.baseUrl = 'http://brawlstars-api.herokuapp.com/api/' - should be self.base_url
• a blank __del__...?
• if tag is None: raise MissingArg('tag') - this is really useless, just don't make it an optional params and python handles it for you
• [ ]

player = Player(data) 
return player

i don't see the point of creating another variable here

• ^^ the 2 above for get_band
• get_player and get_profile should be aliases
• create an __str__
• personally think get_brawlers and get_id is redundant and get_members
• classes related to the above ^^

async client

• most stuff from above
• except Exception: - just do except:
• do an except for JSONDecodeError as i said in sharpbit's commit comments, it's because of heroku
• a LOT of unnecessary coroutines that can just be functions

sidenote

am kinda tired atm so this isn't comprehensive, fix all these and ill look at it again

read this SharpBit/brawlstats#1

I don't think invalidarg or httperror for invalid tag is a good idea, a seperate exception should be raised for that.