1. You could use 'in'. In fact, there are a number of prepositions that could go in this gap. You could use 'of', 'in' or 'with'. You could even have no preposition at all. If this is a test, it's a bad one, because there are several possible answers.
2. You can't use 'experience' as a countable noun here. In this sense, 'I have experience' is similar to 'I have knowledge' or 'I have had practice', and it's an uncountable noun - so it has to be singular.
You can only say 'experiences' when you are using the word in its countable sense, which has a different meaning. An experience is a single event, or something which has happened to you e.g. 'I had many interesting experiences when I was travelling in Africa.'
NB Note that the word " can't " always needs an apostrophe.