Seems like you are using windows. Windows directory separator is \
not /
try this way:
df=pd.read_csv('C:\\Users\\user\\Desktop\\Work\\a.csv')
or
import os
file_path = os.path.sep.join(["C:", "Users", "user", "Desktop", "Work", "a.csv"])
df = pd.read_csv(file_path)
solved Python Cannot find file although the error says it exists