The "6c" is not the input of a test case you fail. The error means that you pass "6c" to parseInt (or similar), but it can be a part of a larger input string.
Inputs of test cases should be visible in test case titles, can you see them there?

Thanks, the plot thickens.

I can get this to pass all the tests, when I put 4I in I get the expected 4I1 but on attempting it I get the following error with similar tests:

java.lang.NumberFormatException: For input string: "6c"
at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)

The above passes the test phase when I manually put 6c in and I get no errors on my IDE.

I think the correct answer should be "4|1". my method returns "4|1" without giving any error.
Perhaps you are counting digits from "4" so you will have "2 digits(4 & |), but in this case the number of digits should be "0 digits".
I hope it helps.

Can someone tell me what the answer to passing ("4I") should be please?

I thought it should be 4I1 but it is giving an error on that answer with out of bounds (despite working on my IDE) making me think I have got the answer wrong.