all-pole IIR filters

The name of the ENTITY (line 86) is "lowpass"; the matching END (line 99) says "butterworth3".

Technically, both must be the same. The FPGA synthesizers I have tried so far have all failed.

When I updated line 99 to be "END lowpass ;", I had no problem completing synthesis.

Perfectly right I corrected the VHDL

This is right: I corrected the VHDL entity.