Good news is decimal to binary and binary to decimal seem to work fine. The only one that seems to have a problem is decimal to hexadecimal.
If you make the changes where I added comments it should compile.
#Get number from user
number=int(input("enter a postive number ")#Missing a parenthesis
if(number is<= 0) #What's that is doing there? Also, need a colon
print("enter a postive number")
if(number=0) #Should not be an assignment, also a colon
print("the hexadecimal value of 0 equals 0")
else #Colon needed here
else: #This else probably shouldn't be here
print(number, "converted to hexadecimal is " , hstr)
I would also highly recommending downloading a good python IDE - I currently use PyCharm and it works pretty well.