The Pleo category (same as an expense account in Fortnox) requires to also have a Fortnox cost centre (tag in Pleo) connected to the expense.
There are a few alternative solutions; please choose what's best depending on your own accounting setup.
In Pleo, open the specified expense, delete the affected category, and choose a different category (that doesn't require a cost centre).
In Pleo, add a tag that reflects a cost centre. Learn more about tags.
Keep the Pleo expense as-is, but change the expense account setup in Fortnox. Click "Register" → "Kontoplan" (chart of accounts) → click on the affected account → click “Ytterligare utgifter” (additional settings). Under "cost centre", change the selected option from "Not allowed" to "Allowed". Click "Save".