Ugh, sorry to hear that.
Alas, there is currently an issue with the last level, where it can get too complex and affect the pathfinding (I'm finishing up an update rn that should hopefullly resolve this).
Regarding not being able to build units - that's (kinda) by design. There is now a max number of units and buildings and, when reached, neither player nor AI can build until the no. of units drops (e.g. one gets destroyed). However, you should be seeing a message at the top informing you that no more can be built (perhaps this still wasn't clear tho?).
In saying that, this "race to build units faster than the AI" will hopefully get better in the next release - as the AI will give you a bit more time to get your base established.
Thanks again for the bug report!