Total value of the entries within the initial UTxO set (the amount of money available) is less than the the total value of all entries in the requested output set (the amount of money required).
The algorithm depletes all entries from the initial UTxO set before it is able to pay for all outputs in the requested output set. This can happen even if the total value of entries within the initial UTxO set is greater than the total value of all entries in the requested output set, due to various restrictions that coin selection algorithms impose on themselves when selecting UTxO entries.
The number of entries in the initial UTxO set is smaller than the number of entries in the requested output set, for algorithms that impose the restriction that a single UTxO entry can only be used to pay for at most one output.
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Another input must be selected by the algorithm in order to continue making progress, but doing so will increase the size of the resulting selection beyond an acceptable limit, specified by the maximum input count parameter.