You have one too many slashes in your s/// (you have 4, but you can only use 3). Either backslash one of the 2 in the middle, or choose different delimiters:

s/^Alert Level\s0*(?:[3-9]|\d{2,})/(?!.*Log Entry)/DEREK!!!!\n$&/s;
 ^                                ^               ^             ^
 |                                |               |             |
 |                                |               |             |
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