Transceiver fails to go into transmit when keyed. Receive works great.

Put SWR meter between your antenna and output, test wave length receive/transmit, might need a ground plane or center loaded stick.
Could be transceiver relay, try clicking mic fast, it should click out as fast as you key the mic.

If your receiver was operating previously but now the SSB option is not working, very likely the sideband detector has failed. I'm not directly familiar with your Yaesu unit but, in most Yaesu transceivers, the SSB chip and control circuit (including diodes) is identified in print on the main board.

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The radio has been locked out of service by the owner. Return it to get it unlocked:

Pg 35 of the manual:
D D Stun and Revive
The dispatcher can send MDC 1200 signals that will stun or
revive your transceiver. If a Stun command is received that
matches your station ID, the transceiver will display "SORRY"
and you can not receive or transmit. When a Revive com-
mand is received that matches your station ID, normal opera-
tion is restored.

Users Manual (download if you don't have it)
TX I
NHIBIT
You can inhibit the transmission to prevent unauthorized
individuals from transmitting, or to eliminate accidental
transmissions while carrying the transceiver.
1 Press [MENU].
2 Turn the Tuning control to select Menu No. 25 (TXI).
3 Press [MENU].
4 Turn the Tuning control to select "ON".
5 Press [MENU] to store the setting or [PTT] to cancel.
•
"TX INH" appears and an error beep sounds if the
transceiver tries to transmit while TX Inhibit is ON

try to set them to a same channel via the dial on top. If that does not work - one walkie may be dead ...

restore.

Probably the switching transistor circuit in the main board.

The Yaesu FT-8900 is a quad band FM transceiver only. It can not be modified to receive or transmit AM, SSB, RTTY or CW. It works on all or portions of 70 centimeter, 2 meter, 6 meter and 10 meter bands. The 26.805 Mhz frequency is 11 meters, so it is out of range of the 10 meter band in the radio, it is close. Even if able expand the transceiver, the AM / SSB mode commonly found in this band makes it impossible to use. Further, the radio is not FCC Type Accepted for use in this band.

Model #? Name?
black box? make sure the tx rx are reverced on both ends.