This error occurs even though the Timed Loop (Single-Cycle)
"For Loops in Single-Cycle Timed Loops: You can place a For Loop in a single-cycle Timed Loop if the For Loop contains Numeric, Boolean, or Comparison operations and uses only auto-indexed tunnels. Objects that generate or contain state, such as shift registers, Feedback Nodes, or VI calls, are not allowed in For Loops within a single-cycle Timed Loop."
This is a known issue and a CAR (#592391) has been filed for this. You can watch the Fixed Issues list when new versions of the LabVIEW FPGA Module are released to check if this has been resolved. Use the workaround outlined in the Solutions section if you need to use a Select structure.