This is collection of popular errors made when entering FriCAS expressions.

Missing parenthesis

sin(x

  Line   1: sin(x
           ...AB
  Error  A: Missing mate.
  Error  B: syntax error at top level
  Error  B: Possibly missing a ) 
   3 error(s) parsing

sin(cos(x)

  Line   1: sin(cos(x)
           ...A.....B
  Error  A: Missing mate.
  Error  B: syntax error at top level
  Error  B: Possibly missing a ) 
   3 error(s) parsing

sin cos(x))

  Line   1: sin cos(x))
           ..........A
  Error  A: Improper syntax.
   1 error(s) parsing

No unary +

+a*a

   There are no library operations named + having 1 argument(s) though 
      there are 13 exposed operation(s) and 11 unexposed operation(s) 
      having a different number of arguments. Use HyperDoc Browse, or 
      issue
                                 )what op +
      to learn what operations contain " + " in their names, or issue
                                )display op +
      to learn more about the available operations.

   Cannot find a definition or applicable library operation named + 
      with argument type(s) 
                             Polynomial(Integer)

      Perhaps you should use "@" to indicate the required return type, 
      or "$" to specify which version of the function you need.

No ** (use ^ instead)

a^2

(1)

a**2

   There are no library operations named ** 
      Use HyperDoc Browse or issue
                                 )what op **
      to learn if there is any operation containing " ** " in its name.

   Cannot find a definition or applicable library operation named ** 
      with argument type(s) 
                                 Variable(a)
                               PositiveInteger

      Perhaps you should use "@" to indicate the required return type, 
      or "$" to specify which version of the function you need.

FriCAS needs explicit multiplication:

(a-3*b)(x-a+3*b)

   There are 1 exposed and 1 unexposed library operations named elt 
      having 1 argument(s) but none was determined to be applicable. 
      Use HyperDoc Browse, or issue
                               )display op elt
      to learn more about the available operations. Perhaps 
      package-calling the operation or using coercions on the arguments
      will allow you to apply the operation.

   Cannot find application of object of type Polynomial(Integer) to 
      argument(s) of type(s) 
                             Polynomial(Integer)

Double =, accepted but gives nonsense type

(a-3*b)*(x-a+3*b) = 0= 0

(2)