The use of 放在 in a sentence
The first sentence &quot;我把雪糖放在这杯咖啡里&quot; is grammatically correct

[我 - subject (I)]
[把 - disposal marker that dispose the following object ]
[砂糖 - object (sugar)]
[放 - verb (put)]
[在( ~ )里 - preposition (into)]
[在(这杯咖啡)里 - relative phrase indicates location (into this cup of coffee)]

The second sentence &quot;我放在这杯咖啡(砂糖)&quot; is grammatically incorrect.

It should be: [我放 + (object)+  在这杯咖啡里] = &quot;我放(砂糖)在这杯咖啡里&quot;
'砂糖'(object) is wrongly placed after the (relative phrase that indicate location) '这杯咖啡里'. That created a '[这杯咖啡 (n)]+[砂糖 (n)] situation'
it also missed the '里' in the preposition '在 ~ 里'

to put the object '砂糖' after  '这杯咖啡' , you cannot use the disposal  marker '把'
The correct form is &quot;我在这杯咖啡里放入砂糖&quot;

我(subject)
在( ~ )里(preposition)
在(这杯咖啡)里 (relative phrase- indicate location)
放入 (verb)
砂糖 (object)