I'd start with this link for your syntax.
https://experienceleague.adobe.com/en/docs/workfront/using/reporting/reports/calculated-custom-data/...
For example the ISBLANK syntax is "ISBLANK(value)" -- it's not "ISBLANK(value, value2, value3, and so on)"
Because I hate building these types of chains (it's so menial...), I also recommend going through AI (e.g. copilot) to help automate an answer. I'm not saying the below is necessarily accurate, but it's a start at showing you potentially what syntax you should be looking at. AI even helpfully throws in an "in Progress" answer in case some smart aleck starts filling out the fields backwards.
Remember to put in all your "DE:"s in front of your field names.
IF(ISBLANK({Field A}),
"Start Checklist",
IF(!ISBLANK({Field A}) && ISBLANK({Field B}) && ISBLANK({Field C}) && ISBLANK({Field D}) && ISBLANK({Field E}) && ISBLANK({Field F}),
"I have completed First check",
IF(!ISBLANK({Field A}) && !ISBLANK({Field B}) && ISBLANK({Field C}) && ISBLANK({Field D}) && ISBLANK({Field E}) && ISBLANK({Field F}),
"I have completed check 1 and 2",
IF(!ISBLANK({Field A}) && !ISBLANK({Field B}) && !ISBLANK({Field C}) && ISBLANK({Field D}) && ISBLANK({Field E}) && ISBLANK({Field F}),
"I have completed check 1 to 3",
IF(!ISBLANK({Field A}) && !ISBLANK({Field B}) && !ISBLANK({Field C}) && !ISBLANK({Field D}) && ISBLANK({Field E}) && ISBLANK({Field F}),
"I have completed check 1 to 4",
IF(!ISBLANK({Field A}) && !ISBLANK({Field B}) && !ISBLANK({Field C}) && !ISBLANK({Field D}) && !ISBLANK({Field E}) && ISBLANK({Field F}),
"I have completed check 1 to 5",
IF(!ISBLANK({Field A}) && !ISBLANK({Field B}) && !ISBLANK({Field C}) && !ISBLANK({Field D}) && !ISBLANK({Field E}) && !ISBLANK({Field F}),
"All checks completed",
"In Progress"
)
)
)
)
)
)
)
By the way, the best way to get an accurate answer out of AI is to show it something that works. So even if you can get something small working, like field A is not blank and field B is blank -- AI can more successfully extrapolate up to field F for you.