All the docs and error messages seem to indicate that you need a tuple as the object in “”.join but it seems an anonymous list works as well – and format works inside the process also.

str1="The value of pi, "
pi = 3.14159265359
str2=", can be estimated to 5 decimal places as "
str3="."

str4="".join(list(str1 + str(pi) +  str2 + str(format(pi, ".5f")) + str3))

print(str4)

Results in:
The value of pi, 3.14159265359, can be estimated to 5 decimal places as 3.14159.

(Yes, I am aware that pi is irrational so no need to “call in” this one.  And yes it can all be done much neater with an f string:

print(f"{str1}{str(pi)}{str2}{pi: <.5f}{'.'}")

)