Yes, forks are possible in Proof of Stake.
Here is an example. Assume Alice (A) is the proposer for slot 1 and Bob (B) for slot 2. The genesis (G) block is slot 0.
Alice creates the block at slot 1. This chain is G->A.
Bob never saw Alice's block so he creates the block at slot 2. This chain is G->empty->B.
There's now 2 forks. Charlie, the proposer ...