Why can I not enter an SPX order in a penny increment?

SPX trades in specific increments

The tastytrade trading platform will adjust single-leg SPX options orders in increments of $0.05 when premiums are less than $3 and $0.10 for premiums higher than or equal to $3. However, when trading multi-leg spreads, they trade in $0.05 increments. tastytrade allows the price to be modified by the because there may be times when the CBOE makes changes to their minimum pricing increments. As a result, locking in these programmed pricing steps would prevent order entry.

Why is my order SPX order auto-canceling or rejecting?

Check your price increment

Orders entered with a price that the CBOE does not accept will be automatically canceled or rejected by the CBOE execution broker. When an SPX order cancels on its own or gets rejected, then it means that an incorrect price increment was entered. For example, the order routed in a penny ($0.01) increment.