The test is: does it repeat?
Not whether the task is annoying. Whether it happens again, on a schedule or a trigger, in roughly the same shape each time. The Friday supplier payment run repeats. Cleaning up one bad data import happens once. Automate the first. Do the second by hand and move on.
A one-off is cheaper by hand
Automating something you will do once means building it, testing it, and writing it down, for a tool that then has no second use. That is almost always slower than just doing the task.
The exception is a one-off so large it is really thousands of small repeats, like migrating ten thousand records. There the record is the unit that repeats, and automation earns its place.
Repeatable work adds up, both ways
A process you run fifty times a year is fifty chances to skip a step, fifty evenings someone loses to it, fifty places a number gets mistyped. Automate it once and all fifty improve together. Leave it manual and that cost is paid again every month, quietly, for as long as the business runs.
What to automate first
Not the hardest task. The one that repeats, runs on clear rules rather than judgement, and is currently held up by one person. An invoice raised from a delivery note. A weekly report someone rebuilds by hand. The reminder that only goes out when somebody remembers.
Start there, prove it works, then take the next one. If you are not sure which of your processes clear that bar, our free operations check lists the twelve we look at first. Most of the wins are duller and closer than people expect.