Will 208 volts work in the place of 240 volts on a ac unit

A lot depends on what type of set up your talking about. Is there a phase difference in the setup? If so then it can't be done. Unless there are ways (diagrams from manufacturer) for setting up at different voltages. But most likely its not going to work.

Most equipment, including refrigeration compressors, can can handle 10% plus or minus tolerance of voltage. 240 Volts minus 10% is 216 volts.

If your unit is rated 240 volts, then 208 volts may not work.

Most compressors are actually rated 230 volts, if yours is, then 208 will be just enough to work properly.

Since we do not know exactly what you have, you need to refer to the manufacturers specifications of your unit.