"Overissue" means literally "to issue too much". That is, to make available too much of something. The most common usage would be in finance, where a company or government might "Overissue" bonds or currency, meaning they make too much available to the market and problems result.

Over- is a common prefix - it can be used with verbs adjectives adverbs and nouns, and has quite a few senses. The sense of 'over' in this case is 'in excess' - as it is also in 'overkill' 'overdo' 'overcrowd' 'overdose' ... Sometimes it is hyphenated (as over-issue is in my Oxford dictionary, or over-excite)