You usually can't just butt two formulas end-to-end and have it work.
construction is not valid syntax. Formulas need to end up providing a single value in the end, and each of your top-level
IF() functions provide their own value. So the
IF(COUNTRY = 'DE', ...) is the "extra IF" the error is complaining about.
In some situations, you could use an operator to take those two results and combine them into a single result, e.g.
/* Both IF()s return a single integer */
/* We use addition to take those two intermediate results, and */
/* combine them into the final, single result */
IF(COUNTRY = 'DE', 1, 0) + IF(CONTAINS('DE|IE'), 2, 0)
In your case, what you want to do is to nest your second
IF() into the "expression if false" part of the first
IF(). After all, you don't want to return null unless the Country is neither 'GB', nor 'IE', nor 'DE'. The Country being neither 'GB' or 'IE' is not sufficient for you to give
null as the result.
/* You only want a single top-level IF() */
/* begin value/expression when true */
EMPLOYEES >= 750,
/* and I'm not typing out the rest */
/* end value/expression when true */
/* begin value/expression when false */
/* Instead of returning null, you need to further check to see if */
/* the Country is DE */
COUNTRY = 'DE',
/* not gonna type out the rest of this either */
/* end value/expression when false */