This may help make things clearer:
#show-pins | sort
P8.03 / eMMC d6 6 fast rx up 1 mmc 1 d6 mmc@481d8000 (pinmux_emmc_pins)
P8.04 / eMMC d7 7 fast rx up 1 mmc 1 d7 mmc@481d8000 (pinmux_emmc_pins)
P8.05 / eMMC d2 2 fast rx up 1 mmc 1 d2 mmc@481d8000 (pinmux_emmc_pins)
P8.06 / eMMC d3 3 fast rx up 1 mmc 1 d3 mmc@481d8000 (pinmux_emmc_pins)
P8.07 36 fast 7 gpio 2.02 lo >> leds (pinmux_led_pins)
P8.08 37 fast 7 gpio 2.03 lo >> leds (pinmux_led_pins)
P8.09 39 fast rx up 7 gpio 2.05 << hi sysfs (pins)
P8.10 38 fast rx up 7 gpio 2.04 << hi sysfs (pins)
P8.11 13 fast rx up 4 qep 2 in B ocp/helper (pins)
P8.12 12 fast rx up 4 qep 2 in A ocp/helper (pins)
P8.13 9 fast down 4 pwm 2 out B ocp/helper (pins)
P8.14 10 fast 7 gpio 0.26 lo >> leds (pinmux_led_pins)
P8.15 15 fast rx up 6 pru 0 in 15 ocp/helper (pins)
P8.16 14 fast rx up 6 pru 0 in 14 ocp/helper (pins)
P8.17 11 fast 7 gpio 0.27 lo >> leds (pinmux_led_pins)
P8.18 35 fast 7 gpio 2.01 ocp/helper (pins)
P8.19 8 fast down 4 pwm 2 out A ocp/helper (pins)
P8.20 / eMMC cmd 33 fast rx up 2 mmc 1 cmd mmc@481d8000 (pinmux_emmc_pins)
P8.21 / eMMC clk 32 fast rx up 2 mmc 1 clk mmc@481d8000 (pinmux_emmc_pins)
P8.22 / eMMC d5 5 fast rx up 1 mmc 1 d5 mmc@481d8000 (pinmux_emmc_pins)
P8.23 / eMMC d4 4 fast rx up 1 mmc 1 d4 mmc@481d8000 (pinmux_emmc_pins)
P8.24 / eMMC d1 1 fast rx up 1 mmc 1 d1 mmc@481d8000 (pinmux_emmc_pins)
P8.25 / eMMC d0 0 fast rx up 1 mmc 1 d0 mmc@481d8000 (pinmux_emmc_pins)
P8.26 31 fast 7 gpio 1.29 lo >> leds (pinmux_led_pins)
P8.27 / hdmi 56 fast down 5 pru 1 out 8 ocp/helper (pins)
P8.28 / hdmi 58 fast down 5 pru 1 out 10 ocp/helper (pins)
P8.29 / hdmi 57 fast down 5 pru 1 out 9 ocp/helper (pins)
P8.30 / hdmi 59 fast down 5 pru 1 out 11 ocp/helper (pins)
P8.31 / hdmi / sysboot 14 54 fast 7 gpio 0.10 lo >> sysfs (pins)
P8.32 / hdmi / sysboot 15 55 fast 7 gpio 0.11 lo >> leds (pinmux_led_pins)
P8.33 / hdmi / sysboot 13 53 fast rx up 2 qep 1 in B ocp/helper (pins)
P8.34 / hdmi / sysboot 11 51 fast rx 7 gpio 2.17
P8.35 / hdmi / sysboot 12 52 fast rx up 2 qep 1 in A ocp/helper (pins)
P8.36 / hdmi / sysboot 10 50 fast 7 gpio 2.16 lo >> sysfs (pins)
P8.37 / hdmi / sysboot 8 48 fast 4 uart 5 txd ocp/helper (pins)
P8.38 / hdmi / sysboot 9 49 fast rx 4 uart 5 rxd ocp/helper (pins)
P8.39 / hdmi / sysboot 6 46 fast down 5 pru 1 out 6 ocp/helper (pins)
P8.40 / hdmi / sysboot 7 47 fast down 5 pru 1 out 7 ocp/helper (pins)
P8.41 / hdmi / sysboot 4 44 fast down 5 pru 1 out 4 ocp/helper (pins)
P8.42 / hdmi / sysboot 5 45 fast down 5 pru 1 out 5 ocp/helper (pins)
P8.43 / hdmi / sysboot 2 42 fast 7 gpio 2.08 lo >> sysfs (pins)
P8.44 / hdmi / sysboot 3 43 fast 7 gpio 2.09 lo >> sysfs (pins)
P8.45 / hdmi / sysboot 0 40 fast 7 gpio 2.06 lo >> sysfs (pins)
P8.46 / hdmi / sysboot 1 41 fast 7 gpio 2.07 lo >> sysfs (pins)
P9.11 28 fast rx up 6 uart 4 rxd ocp/P9_11_pinmux (pinmux_P9_11_uart_pin)
P9.12 30 fast 7 gpio 1.28 ocp/helper (pins)
P9.13 29 fast 7 gpio 0.31 lo >> sysfs (pins)
P9.14 18 fast down 6 pwm 1 out A ocp/helper (pins)
P9.15 16 fast 7 gpio 1.16 lo >> sysfs (pins)
P9.15 34 fast 7 gpio 2.00 lo >> sysfs (pins)
P9.16 19 fast down 6 pwm 1 out B ocp/helper (pins)
P9.17 / spi boot cs 87 fast rx up 2 i²c 1 scl ocp/helper (pins)
P9.18 / spi boot out 86 fast rx up 2 i²c 1 sda ocp/helper (pins)
P9.19 / cape i²c scl 95 slow rx up 3 i²c 2 scl ocp/helper (pins)
P9.20 / cape i²c sda 94 slow rx up 3 i²c 2 sda ocp/helper (pins)
P9.21 / spi boot in 85 fast rx up 1 uart 2 txd ocp/P9_21_pinmux (pinmux_P9_21_uart_pin)
P9.22 / spi boot clk 84 fast rx up 1 uart 2 rxd ocp/P9_22_pinmux (pinmux_P9_22_uart_pin)
P9.23 17 fast rx up 7 gpio 1.17 ocp/P9_23_pinmux (pinmux_P9_23_gpio_pu_pin)
P9.24 97 fast rx up 0 uart 1 txd ocp/P9_24_pinmux (pinmux_P9_24_uart_pin)
P9.25 / audio osc 107 fast rx up 7 gpio 3.21 << lo sysfs (pins)
P9.26 96 fast rx up 0 uart 1 rxd ocp/P9_26_pinmux (pinmux_P9_26_uart_pin)
P9.27 105 fast rx up 1 qep 0 in B ocp/helper (pins)
P9.28 / hdmi audio data 103 fast rx up 7 gpio 3.17 << hi sysfs (pinmux_P9_28_gpio_pu_pin)
P9.29 / hdmi audio fs 101 fast rx down 3 spi 1 d0 miso ocp/P9_29_pinmux (pinmux_P9_29_spi_pin)
P9.30 102 fast rx down 3 spi 1 d1 mosi ocp/P9_30_pinmux (pinmux_P9_30_spi_pin)
P9.31 / hdmi audio clk 100 fast rx down 3 spi 1 clk ocp/P9_31_pinmux (pinmux_P9_31_spi_pin)
P9.41 106 fast rx up 7 gpio 3.20 ocp/D13_pinmux (pinmux_D13_gpio_pu_pin)
P9.41 / jtag emu3 109 fast 7 gpio 0.20 lo >> sysfs (pins)
P9.42 104 fast rx up 1 qep 0 in A ocp/helper (pins)
P9.42 89 fast 2 spi 1 cs 1 ocp/C18_pinmux (pinmux_C18_spi_pin)
Instead of:
ocp/helper (pins) or sysfs (pins)
it's expecting something akin to:
ocp/P8_45_pinmux (pinmux_P8_45_xxxxx)