the founders would commit themselves to invest in it some fixed amount of money - that would be "preuzeti poslovni udjeli" (= "subscribed capital" one possible translation)
so "preuzeti poslovni udjeli" will only be "a promise to invest" (puko obećanje), nothing more.
when the money is effectively put into the company, that would be "unos" - what exact term would be used would depend on the context - "capital contribution" is one possibility)
so:
"Ulozi za preuzete poslovne udjele uplaćuju se samo u novcu"
could be explained as:
according to this law "when you put into the company the amount of capital you promised, you can do it only in form of money (not in any other form, you can't "contribute to the capital" with your car or your house, or some patent you hold ...)
The exact wording of the translation would have to be adapted to the terminology used for the rest of the text.
more
https://www.quora.com/What-is-a-subscribed-capitalhttps://www.quora.com/What-are-“fully-paid-up-shares”but this doesn't necessarily apply to a simple legal form as is "jednostavno društvo s ograničenom odgovornošću" in all details.