how to auto-assign the pins

Of course of course.

Maybe the tool cant do this because of the warning I got from the tool.

Do you think the tool can not assign pins because it gives critical warning and cant find any pll for the required bank.

===============================================

Warning: Some pins have incomplete I/O assignments. Refer to the I/O Assignment Warnings report for details

Critical Warning: No exact pin location assignment(s) for 153 pins of 153 total pins

...............................

Info: Pin iTile4SerClk not assigned to an exact location on the device

...................................

Error: Can't place MPLL or GPLL PLL "mainpll:mainpll_inst|altpll:altpll_component|mainpll_altpll:auto_generated|pll1" in target device due to device constraints