Tax credits cannot be added to the bill due to the Tax Credit button being greyed out. The tax credit button is used for value added tax or goods and services taxes where tax on purchases can be used to offset tax collected on sales (ie: GST, HST, VAT).
Figure 1 below shows the greyed out tax button circled.
The tax credit button will be greyed out if the supplier on the bill has a tax exemption for the GST/HST/VAT. Review the supplier account to ensure the supplier should have their purchases taxed. The tax settings are found on the financial tab of the supplier record as shown in figure 2 below.
Notice in figure 2 that the tax 2 is not activated. If this tax setting is activated and we open the bill, then the tax credit button will appear again. To do this, the checkmark will be placed in the tax 2 area, then click the OK button to save the change made on the supplier's account. Figure 3 shows the selected supplier tax setting.
Edit the bill again and the tax credit button should be active again as shown in Figure 4.