The AMD Vivado™ Integrated Design Environment (IDE) uses Xilinx design constraints (XDC), and does not support the legacy user constraints file (UCF) format.
Understand that XDC constraints differ significantly from UCF constraints. XDC constraints follow the standard Synopsys design constraints (SDC) format. SDC has evolved for more than 20 years, making it the most widely used and proven format for describing design constraints.
If you already know UCF but are new to XDC, review Differences Between XDC and UCF Constraints in the ISE to Vivado Design Suite Migration Guide (UG911). This chapter also describes how to convert existing UCF files to XDC so you can use them as a starting point for creating XDC constraints.